Alternate Character Interpretation: Plenty of people, even some in-game characters, just plain don't like the "good guy" Archmage Traven. Your interpretation of him and his apparent grudge against necromancy colors the Mages Guild quest line; from what it seems, while the necromancers are truly a threat, at least part of the Guild's fervent opposition to them is due to Traven's personal agenda.
There's also the fact that he closed the Imperial University and eliminated freedom of research, which is antithetical to the reasons the Guild was founded in the first place. Any more than a cursory glance has Traven as a man with a grudge against necromancy and is willing to do anything to destroy it, rather than an archmage doing what's best for the guild. It certainly doesn't help that most of the smaller guildhalls are run incompetently by Know-Nothing Know-It-Alls or people who were Kicked Upstairs.
It's mentioned several times that he severely weakened the Mages Guild. By the time of Skyrim it no longer exists, likely collapsing during the ensuing chaos resulting from the Septim bloodline ending
Mankar Camoran has a few odd things about him that makes him suspect to many players. Most notably, the historical Camoran wasn't an Altmer, while the one in-game is. He confuses several Daedric Princes and their realms. He's wearing the Amulet of Kings despite not being related to the imperial family (though events in Skyrim could explain this). Many players suspect he's a false front for the Mythic Dawn.
The Society for Concerned Merchants is an organization in the Imperial City formed by the Market District's shop owners, to promote a "nice, fair economic balance in town." A quest started by the society's chairperson, Jensine, has the players investigating a new business owner, Thoronir, who's merchandise is suspiciously inexpensive. It turns out that he'd been unwittingly getting his stock from a grave robber. The quest ends with Thoronir vowing to be more careful about his suppliers and joining the Society, and the game treats this like a happy ending, but it doesn't take much twisting to interpret the Society for Concerned Merchants as a group of price-fixers that sought to oust Thoronir for undercutting them.
According to lore, the Blades are extremely competent spies, yet the Emperor is assassinated on their watch, all of their important secrets have been leaked to the Mythic Dawn, the Amulet of Kings is stolen because the Grandmaster left the room to pray, Baurus abandons the Player for no good reason during the most important part of the investigation, and their agents spend most of the game safe at Cloud Ruler Temple while relying on the Player to complete dangerous missions (the success of which determines the very fate of Tamriel) without any help.
Their fanatic devotion to the Septim dynasty, and only the Septims (with Jauffre even stating that the Blades would never be loyal to a non-Septim like Ocato), is also cause for concern.
Anticlimax Boss: Agronak Gro-Malog, the Grand Champion of the Arena. Agronak's level isn't scaled to the character level so at mid level and above he is easier than the mooks you just fought in the Arena. Of course, all the NPCs still react to the fight as if beating him was a huge accomplishment. You can even beat him at low level if you do his quest.
Erandur-Vangaril, a lich with a backstory that has a bug that causes it to always have only 15 Health, and you can only fight it if you're at least level 23, at which point you can almost certainly kill it in one hit.
Mannimarco too. His mooks are slow and avoidable, he has no game in melee combat, and if you have a decent weapon, then he can be killed with a dozen or so attacks at the most.
He becomes even more of one for a player of Daggerfall, who likely would have expected Mannimarco, the King of Worms to look like the King of Worms (imposing, clad in a red robe covering his face in shadow, eyes glowing brightly). Oblivion's incarnation... is a fairly ordinary-looking Altmer.
Mehrunes Dagon himself. Though not technically a boss battle since you're not supposed to fight him, he is rather anti-climactic. After all the buildup of him coming into the mortal plane, all he does when he eventually appears is stand there and half-heartily stomp on anyone that gets near him. He won't chase the player or anything, and running past him takes almost no effort or proper timing.
Complete Monster: Besides Mannimarco (see The Elder Scrolls), there is Mankar Camoran, the leader of the Mythic Dawn cult and a worshipper of MehrunesDagon. During Oblivion, Mankarís goal is to summon his master into Tamriel so Dagon may destroy the world and create a new one in its place, one that coheres with Camoranís Social Darwinist beliefs. Not only does Mankar aide Dagonís Daedra in attacking the mortal world, he also orchestrates the assassination of Emperor Uriel Septim, as well as his children. He even had the city of Kvatch completely destroyed in a failed attempt to kill Urielís bastard son, Martin. While Mankar lures in his followers with promises of a paradise for their faithful service, this turns out to be a lie. Seemingly idyllic on the surface, Mankarís ďParadiseĒ is really a hell where his followers are constantly killed by monsters, only to be resurrected and killed over and over again. His promise to allow any follower who loses faith to leave his paradise also proves false, as the grotto, which is said to be an exit from the afterlife, really leads to a torture chamber where the unfaithful are made immortal, locked into gibbets, and then dumped into lava. One follower who lost faith was forced by Camoran into inflicting this punishment on his friends over and over again. Paradise is said to be Mankarís vision of how he believes the world will be once Dagon takes over.
To be more specific: Will-o'-the-Wisps, who are, count 'em: magic resistant, fast, turn invisible, can only be hurt with silver/magic weapons, drain stats and can make your weapons/armor rot off your body. And Spider Daedra are literally giant deadly demonic spider-women. Who spawn miniature versions of themselves that paralyze you, so they fit the trope too.
Designated Hero: If you think about, Traven's not a very good Archmage. His very first act upon taking the position is completely outlawing Necromancy in Cyrodiil after centuries of it being tolerated and he expelled and blacklisted over half of the Council when they protested. He forces the Arcane University, one of the greatest learning centers in Tamriel, to close its doors to all but the highest ranking guildmembers, which completely contradicts the guild's original purpose. He's made the guild sever ties with any organization that so much as tolerates necromancy, putting a serious strain on both manpower and resources. Finally, the main reason he seems to hate necromancy is because it involves stealing the souls of living beings, but it's explicitly stated that they didn't start doing this until after Traven's draconian policies came into effect. There is no denying that Mannimarco is evil, but it's all but stated that the only reason his Order has become so powerful is because Traven's actions allowed his ranks to become bolstered by hundreds of disgruntled mages. As stated above, this is why the Guild fell to both the strong anti-magic sentiment that arose after the "Oblivion Crisis" and the Thalmor in the centuries between Oblivion and Skyrim. If it makes anyone feel better, Traven helps you to stop Mannimarco by trapping his soul into the Order of the Black Worm's giant soul gem to protect you and is most likely trapped in the Soul Cairn and must suffer for it.
Once Mythic Dawn agents start openly attacking people, it's possible for one such agent, Cingor, to be rendered not just peaceful, but friendly with a high enough influence in the Fighter's Guild.
One of the rather hilarious ones involves Shadowmere, the horse you can get during the Dark Brotherhood questline. Unlike all other horses, she's marked as essential and can't be killed, only briefly knocked unconscious. It's still possible to access her as if she were a corpse, though, and as with all other bodies, it's possible to place items into her inventory, essentially turning her into a portable storage chest. Furthermore, placing potions into her inventory will cause her to actually drink them during battle, enhancing her already considerable combat effectiveness.
Not useful, but quite hilarious nonetheless. At the Arcane University, casting a frenzy spell on a mage scholar or apprentice will cause them to attack each other and the intervening guards. After the spell has worn out, the hostilities between the scholars and the apprentices still exist and they'll fight to the last person. Because all parties are respawnable, they'll all come back after leaving the university for a while. The end result is a scholar vs. apprentice rumble every time you return from your quest.
When the avatar of Mehrunes Dagon appears in the middle of the Imperial City in the finale of the main storyline, you aren't supposed to fight him, and he has the stats to make sure that you won't. However, the Wabbajack works on him, and if you get lucky with it, you can turn him into something harmless and easily killed, like a sheep. And when you do kill him, he melts.
After the Fighter's Guild mission "Trolls of Forsaken Mine", upon asking for your next mission, you're demoted because the Guild Master blames you and Modryn Oreyn for the death of her son. So you're forced to do an extra mission to regain your rank. However, this only happens if you go to Anvil's Guild Hall to ask for your next mission. You still have to do the extra mission, but you don't lose your rank (so it's really more like extra credit).
If you're suspended from the Mages' Guild, you are required to gather alchemic ingredients for Raminus Polus to get back innote 20 Dragon's Tongues and 20 Redwort Flowers for theft, 20 Vampire Dusts and 20 Daedra Hearts for murder. If it's your first offense, a glitch allows you to get back in if you collect 20 pieces of only one of the required items.
If you jump at just the right moment when running up a set of steps you bug out the physics engine and ramp it, if your athleticism is high enough, or if you say... drink 3 bottles of skooma to give you the effect instead... then ramp... you go flying through the air, while this can get you trapped outside the citie's walls if you're not cautious, it can also be used to beat the main game for the low low price of stealing a few bottles of skooma.(This is done by using the ramping to get on top of the buildings in the Imperial City, Temple districts, then jumping onto the main mbuilding, which has no collision as it isn't otherwise reachable. There's a door there that leads to the invaded version of the city, head through it, the turn back around and watch a scene play... Tadah! You beat the game... at level 1.)
The Ogre has the same soundbytes as the Imp from Doom. Three years after this game's initial release, Zenimax (Bethesda's parent company) acquired id Software, the company responsible for developing Doom.
Ho Yay: From Knights of the Nine, Sir Berich and Sir Caius. Despite having been killed by the former, Sir Caius seems oddly eager to clear Sir Berich's name and enter Afterlife with him. Sir Berich seems equally excited about it.
Since Martin seems to develop a crush on the player, this happens if you're playing a male character.
The blacksmith in Mania will hit on the player if you're a male character. It's worth noting that he's a male Orc who thinks he's a young human girl.
Implied Les Yay with the blacksmith in Dementia as well... she'll hit on the player after becoming Sheogorath, even if you're female.
Mythic Dawn sleeper agents Jearl and Saveri Faram in Bruma have some subtext indicating they arw evil lesbians. They live in the same house and are assigned to the same bed, which they would sleep in together if not for an oversight (it is a single bed, so one character is forced to wander around while the other sleeps). During the quest that involves rooting out the two spies, people around town are more concerned with spreading gossip about the stranger they saw at Jearl's house than any suspicious activity they may have noticed.
The character creation is a common joke, mostly because it's much easier to create a terrifying mutant than an actual human being.
Misaimed Fandom: Mannimarco and his Order of the Black Worm are only a small niche of Necromancers, and yet some fans believe they represent Necromancy as a whole.
Moral Event Horizon: Mannimarco destroying the Bruma guildhall in the Mages Guild questline, The Blackwood Company killing the guildmaster's youngest son in the Fighters Guild questline, and Umaril slaughtering multiple Chapels in Knights of the Nine.
The protagonist can cross this in multiple ways, the most obvious choice being almost any quest offered by the Dark Brotherhood.
Fixing Xedilian normally doesn't sound bad... but consider this: Its standard MO is to either a) kill off greedy adventurers, or b) render them irreversibly insane. Points for the creativity of said Mind Rapes. Hallucinating into believing that the rat you just taunted is now going to murder you, searching a hundred duplicate keys to see which one works, and worst of all, a forced temporary and literal out-of-body experience. But hey, the Shivering Isles always needs residents...
Most Annoying Sound: Including random villagers talking about mud crabs, and shopkeepers telling you how great you are at haggling.
Narm: The random conversations between npc's can sometimes become this. A good example is when they suddenly start talking about mudcrabs and how ugly they are.
Paranoia Fuel: Some may be uncomfortable if you don't know what the "Your killing has been observed by forces unknown" means.
By the time you're done with the Shivering Isles questline, you'll have become very wary of corpses in case they're not quite dead.
Player Punch: The Dark Brotherhood questline contains a very, very nasty example of this. By the end of the quest line, every single member that you knew at the beginning is dead.
Martin in the main quest.
Viranus Dalton and Biene Amelion in the Fighters Guild.
Sheogorath in the Shivering Isles.
The Bruma guildhall, and everyone in it, getting blown up by Mannimarco during the Mages Guild questline.
Seinfeld Is Unfunny: A common complaint from newer Elder Scrolls fans introduced to the series via Skyrim or Online is that Oblivion's mechanics are much more obtuse and obnoxious than the latter games.
Interestingly enough, many fans of Morrowind who had been waiting for Oblivion's release actually found themselves put off by the cumbersome mechanics, flawed level scaling, the "weird" faces which were soft and pudgy compared to Morrowind's sharp, angular models, and the overworld which felt too ordinary in comparison to the rich alien world of Morrowind. Not to mention the removal of beloved (and broken) mechanics such as levitation and scroll-making, many fans felt it was nearly a downgrade from Morrowind. Possibly a case of They Changed It, Now It Sucks, but it seems that Oblivion will always remain the Un Favorite middle sibling between Morrowind and Skyrim.
The Level Scaling mechanics have attracted some of the greatest hate of anything in the entire series to date. It makes exploring at low levels fairly boring (Why go look for a new dungeon in hopes of a cool item when it will have the same exact useless loot guarded by the exact same enemies?) and leads to oddities like being the champion of the arena at level 1 thanks to the fact that skills increase independently from level-ups. In fact, if you're not extremely careful when leveling up, you can find yourself quickly outpaced by your enemies. Many online strategy guides actually recommend resting as little as possible to avoid leveling up entirely.
This same scaling also applies to quest rewards. It's entirely possible to complete a quest at level one and obtain a weapon little better than a butter knife, while if you complete the same quest twenty levels higher, you'll obtain that same weapon in Infinity+1 form. As many such rewards are unique, it leads to putting off those quests or encounters as long as possible in hopes of getting something that remains useful for longer than an hour.
The disposition roulette can be this until you learn how it works, and even then your speech can make certain things Lost Forever, thankfully bribing enough can cover you if you fail.
When Horse Armour - one of the first ever paid pieces of DLC (in a game with a very active modding community no less) - first hit the scene, Oblivion players pretty much lost their collective minds.
Squick: One alchemist asks you about the punishment for necrophilia in Cyrodiil. "No reason, just curious." She'll be very happy if you tell her it's just a fine, even for repeated offenses. What makes it even worse, however, is that she makes the comment that the punishment is "much lighter than Morrowind," leaving the impression that she may be in Cyrodiil because of her... habits. There's even a line from an NPC about seeing a Dark Elf walking out of the graveyard at night with "a silly smile on their face."
Not to mention that, y'know... Your character actually knows what the fine for necrophilia is in Cyrodiil. And you start the game in prison...
That One Attack: Its very possible that you will contract Porphyric Hemophilia when fighting vampires, often without realizing it. Only to realize three days later that the disease has advanced and you've become a vampire. Forcing you to either reload an older save or go on a lengthy quest to cure it.
That One Boss: Umbra (the person), if you don't come properly prepared, is quite possibly the hardest boss in the entire game, she wields the weapon Umbra and wears full Ebony Armor. She deals INSANE amounts of damage and an even worse thing is you have to fight her in a small closed area so you'll easily get stuck in a corner. Therefore, if you plan on going through with the associated quest, it's advised that you get yourself plenty of followers (such as the Jemane brothers, Erthor from the Skingrad Recommendation quest, etc.) to back you up, and the best weapons, armor, and spells you can possibly get your hands on at that point.
That One Sidequest: Collecting 100 Nirnroot plants. "You must have turned every stone in all of Cyrodiil". Though it's not quite as hard as it sounds- there are over 300 nirnroot plants in the game (they can be used in alchemy) so finding 100 isn't too straining. Three of the DLC add-ons include extra nirnroots too- and the one in Deepscorn Hollow respawns every 3 days as it's a "collected" food object on a desk, rather than a plant.
Shadowbanish wine. All you're told is that it is found in abandoned forts around Cyrodiil. There are over fifty forts, only about a dozen of which have the wine in them.
And one of those, Fort Grief, isn't even accessible unless you're doing the associated quest.
In a similar fashion, especially if you're not into random dungeon searching, the Museum of Oddities in the Shivering Isles expansion.
Maglir, the biggest coward in the fighter's guild. Getting to splat him is satisfying. That is, assuming a bug in the game doesn't kick in... sometimes he's not properly removed from the Fighter's Guild faction (may be related to OOO or another mod but it has been reported in vanilla environments) and killing him will get you expelled when you return to Chorrol.
Jeanne Frasoric. The friendly and nice guild mistress of Bruma, who can barely be called "sorcerer" let along "mage" . She seems to only be there because of her social skills. She dies in "A Plot Reveled" but whether or not you feel sorry for her is up to how you see her.
Farwil Indarys. His irritating gameplay-related "qualities" are even lampshaded by the Count of Cheydinhal, Andel Indarys, his own father.
Mannimarco because of his Villain Decay. The mod "Mannimarco Revisited" makes him much more fearsome as does Online.
They Changed It, Now It Sucks: Cyrodiil was previously described to be a very Oriental and Mesoamerican inspired Roman Empire. While the more Oriental and Mesoamerican aspects of Cyrodiil were being downplayed and the Roman ones emphasised in Morrowind, this game completely disregards all of that and turns Cyrodiil into a very standard Medieval European Fantasy, with only vaguely Romanesque bits. Not everyone was happy.
Uncanny Valley: Some of the NPCs stare at you bug-eyed throughout every conversation. Without mods, almost every human face looks blotchy and blurry, like they paired high-poly models with Voodoo-era textures. Zooming in for conversation with any non-Argonian (especially Redguards) immediately reminds you that you're playing a videogame.
Vindicated by History: While Oblivion was critically acclaimed on launch, it feel into the usual Bethesda cycle of "Loved on release, disliked by many as flaws become apparent." However, among the older members of the Elder Scrolls fandom who weren't fond of the changes to Skyrim that they saw as dumbing down, this game has gotten a re-examining by fans who enjoy it more now.
Villain Decay: It hits Mannimarco rather hard. Throughout the Mages Guild questline, he's set up as the Arc Villain of that story, having characters become unnerved or outright terrified by knowing that he's reappeared, there's a pretty good poem-style book about his history to find which sets him up as a powerful evil, at one point, he deals a scarily effective, unexpected blow to the Mages Guild and the only survivor of the attack is terrified and tells you how he, concealed by magic, watched Mannimarco rip out his friend's soul and how he believes the guy actually could see through his magical disguise and pretty much just spared him for fun. Then Mannimarco effectively manages to corrupt and disrupt the Mages Council. By the end of the questline, you seriously get the feeling that everything's going to hell for the Mages Guild if you don't stop him quickly. So, after finding his hideout and slaughtering your way through his minions, he's a moderately powerful Altmer wizard whose spells might be slightly threatening at the very worst.
What an Idiot: Azura's followers went out to kill a Vampire, and NOT ONE of them thought to pack some potions to curse the inevitable diseases, or better yet spells? Riiight...
What Do You Mean, It's Not Political?: In the book "The Eastern Provinces", the author gives a very disapproving critique of the Empire's costly occupation of Morrowind and the Black Marsh, citing how wasteful it is to spend taxpayer money on troops stationed there and how, rather than putting an end to the despicable human right violations (slavery), the Empire is only interested in the valuable black natural resource it can exploit from the lands (ebony), and how keeping on this track will lead the Empire into financial ruin.
What The Hell, Casting Agency?: Seriously, whose idea was it to cast Linda Kenyon as every single female Elf character in the game? Her rough voice might have worked for the Dunmer, like in Morrowind, but it's jarring to hear that exact same voice (with little to no variation to speak of) coming out of all the Bosmers and Altmers as well, regardless of how young or old they are, and makes them sound like they have something nasty caught in their throat.
The Woobie: Antoinette Marie of the Dark Brotherhood.
Hirrus Clutumnus in the Shivering Isles DLC, who's whole existence seems to be laced with soul-crushing misery. He can't even bring himself to commit suicide because being a ghost on the Hill of Suicides would be even worse than the life he leads right now.