These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Creator's Pet: While Wizards are a well-loved class straight from the very first iteration of D&D, they're turned into this whenever Monte Cook's around.
Game Breaker: At high-level power play, a straight wizard played by a sufficiently Crazy-Prepared player is considered to be the most powerful class in the game. While they're more fragile than clerics and druids and can't tank, the arcane-exclusive spells (like Teleport) make up for it.
Other 3rd Edition Classes
Game Breaker: Its spell mechanics are similar to a Wizard (a Game Breaker in its own right), and it has access to every divine spell (Clerics and Druids manage to be incredibly powerful with more limited spell lists). And the class has some useful abilities on the side. Every bit as potent as its fellow full casters. Not to mention, you can take Mystic Theurge (advances casting in two spellcasting classes) with both class's spellcasting tied to Intelligence, the most useful mental ability score.
Ensemble Darkhorse: Because of how well-balanced and unique it was, the Ardent was one of the few fondly-remembered things about Complete Psionic.
Ensemble Darkhorse: Fandom find the other 2 classes in ToM to be awful, if not impossible to use. The Binder however is considered to hit the "Powerful enough to be fun to play, but weak enough to not be a Game Breaker", and easily one of the most supported non-core classes by homebrew (possibly the most outside of Tome of Battle).
Game Breaker: It's one of the Big 6, and stands as one of two members of the Big 6 that can use both Arcane and Psionic abilities (the other being the Psionic-variant Artificer). The Erudite is nearly unmatched in flexibility, as you are able to spontaneously manifest any power you know (though there is a debate on how this is supposed to work due to poor wording).
Tier Induced Scrappy: Due to its limited ability to do anything but heal, Healers tend to get a poor reputation.
Rescued from the Scrappy Heap: Giving them access to Sanctified Spells from the Book of Exalted Deeds gives them a fair amount of additional options in combat other than healing (which is perfectly legal by the rules of Sanctified Spells, as Healers are all good-aligned spellcasters who prepare their spells).
Chaotic Neutral: One of the four possible choices for an Incarnate character, and the weakest of the four (ranged-focus, but the developers kinda screwed up here).
Lawful Neutral: One of the four possible choices for an Incarnate character, and the one closest to being a frontliner.
Neutral Good: One of the four possible choices for an Incarnate character, can be played as a tank or as a psuedocaster via Use Magic Device. Has the best defenses this side of a Full Caster.
Tier Induced Scrappy: While it is a Rogue with psychic powers, the Psychic Rogue is considered better due to not having class features that rely on Psionic Focus. This class is still pretty good, but most people will point you towards the Psychic Rogue.
Psion / Psionicist
Game Breaker: Sits up very high, almost one of the Big 6, but not quite. On the other hand it wins the award for being the most accessible class for committing human rights violations.
Even worse: A TW Fing Ranger outclasses a Complete Warrior Samurai. At least the Oriental Adventures one was decent!
When a class is designed around one of the weakest combat mechanics... (Dual Wielding is arguably best optimised by sheathing one of the weapons)
Tier Induced Scrappy: The early levels are murder on a Shadowcaster. They aren't too powerful late-game either, but they are still capable of contributing to the party.
Tier Induced Scrappy: It ends up being the worst of the three Meldshapers. It's even weaker than a Core Paladin!
Tier Induced Scrappy: Because you can't enchant a Mind Blade like you can a physical weapon, a Soulknife tends to do less damage than a fighter of equal level.
Also, see Samurai, "less options". Having the Whirlwind Attack feat without the ridiculous prerequisites is neat, though.
Base Breaker: By virtue of being a Tome of Battle class. It's also the most overtly pseudomagical of the three, so haters who know what they're talking about hate the Swordsage the most.
Game Breaker/The Scrappy: Was intended to break the game according to the Game Breaker page definition and it can at level 19, instead it breaks the game by being poorly designed. This would be considered a Tier Induced Scrappy except it literally is the only class not to be rated on the tier system for 3.X due to the simple fact that its completely incompatible with the rest of D&D. For starters, as it gains levels, it becomes much harder to use its core mechanics. Only by min-maxing to the extreme can it be viable, and the turnaround point is level 19, at which point it can contribute to combat.
Ensemble Darkhorse: Meepo the Kobold in 3rd Edition. This little lizard-dude, originally just a kill-it-for-stuff encounter in the adventure The Sunless Citadel, was so popular that he made an appearance as an NPC in at least one other adventure, featured in a web-exclusive article in which he became a half-dragon were-velociraptor, and got whisked away to another dimension in which he obtained a magical pump-action shotgun. Truly, Meepo is the pinnacle of koboldian awesomesauce. He even makes a cameo in d20 modern.
As far as races go, the Warforged from Eberron were very well received. The Shifters were also well-liked, but not as much as the Warforged.
As a matter of fact, these two races (and Changelings) were put in as monsters in the first 4E Monster Manual, and Shifters were introduced as a playable race in the second 4e Player's Handbook. Warforged were made playable in any setting thanks to a free(!) Dragon article on Wizards' official site.
Hilarious in Hindsight: The complaints about 4th Edition playing like a video game certainly won't get any better when the corebooks have a playable race called the Dragonborn.
The Forsaken are an entire race of Magnificent Bastards.
Memetic Badass: The Tarrasque, which has become a byword for a nigh-unstoppable monster.
Game Breaker: Numerous in all editions, although 3rd edition is particularly famous for this, due to the sheer volume of various mechanical goodies, provided by its supplements, as well as the generally high power level of the characters. Notable in 4th edition for having at least two discovered before the game was released. Errata has fixed most of 4e's breaking stuff, but not all. Some examples:
Clerics, Druids, and prestige classes related to either had the best of being both linear warriors AND quadratic wizards... and aside from a decidedly lower-tier class selection, they were the only healers you could pick. If the Druid or Cleric stuck to healing, it didn't affect party balance. It was when the power gamer got his or her hands on them that it became a problem...
If they stick to healing, it breaks the game in the opposite direction. Attack and Damage scale up far faster the Armor Class and Hit Points in D&D. Playing a Cleric that is The Load isn't much better than playing one that that is the Game Breaker.
Certain builds were able to incur trillions of damage in one attack, at range. One low-level spell with a specific combination of metamagic feats would raze anything and everything in a 10-mile radius to the ground. And then there's Pun-Pun, a level one kobold with inifinite stats who can reach infinitely far, including across the planes, and can cast any and every spell an infinite number of times per day. Did I mention this kobold is more of a god (infinite divine ranks) than the actual gods?
Essentials. Not in-and-of themselves, mind you, and not so much if you're in an all-Essentials group, but consider the following. Prior to the release of Essentials, basic attacks were just that, basic. They were usually the weakest hitting things for damage dealers, and didn't have all the cool effects other powers did. Thus, classes such as Bards, Warlords and the like which gave out basic attacks like candy would only marginally influence the tide of battle. Now enter the Essentials, which are based around improving or replacing basic attacks: Slayers that wield 2-handed weapons that deal as much as 4 weapon damage on a single basic attack; Scouts that get granted one melee attack and are suddenly making 3 or more of them; latter additions give us Vampires that effectively become unkillable as long as they keep hitting, and the Bladesinger that adds a burst of various magical damage each time he hits. Yikes.
Erudites with the "Convert Spell to Power" alternate class feature. Regular Erudites can learn every psionic power there is and use them spontaneously for as long as their Mana Meters hold out. Spell-to-Power Erudites can do that, too...plus they can also learn every arcane spell in the game and convert them to psionic powers, which means they can "cast" them using the same mana meter system and ignore both arcane spell failure and expensive material components. A Spell-to-Power Erudite who can learn 9th Level spells is basically a demigod.
The Illithid Savant prestige class (although, honestly, if your DM lets you play as a mind flayer and also lets you take a prestige class clearly designed for NPC use, he deserves what he gets). Basically, the Illithid Savant is like Sylar, gaining the powers, special abilities, and even spellcasting of those whose brains he eats. (For extra cheese, eat the brain of a Sorcerer or Wizard who can cast Gate. Now if you want a particular ability, just summon up the creature who has it virtually at will.)
Tier Induced Scrappy: 3.5 Monks are noted for getting many abilites that are either done better with spells gotten much quicker (over 20 levels they get the ability to reduce fall damage, that's a first level spell for a caster or 2000 gold for everyone else) and don't synergize at all (Monks have one ability that makes them move fast, and another that requires them to stand still) to the point that when asked to optimize a Monk, most suggestions are to play another class. Truenamers also get a lot of flack for getting weaker as they level up as they need to meet an (already hard check) with a requirement that goes up by 2 each level, but you only get + 1 to make the check a level (Soulknife and CW Samurai also fall into this for similar reasons, but Monk gets the worst because it is "core" and part of the base game and truenamer is just that... egregious).
Complete Warrior Samurai deserves special mention in that is the absolute lowest Tier. In original outline of the various Tiers, CW Samurai is so low that it is actually ranked lower than Expert, an NPC-only class with versatile skill selection and no class features. There is literally nothing that a Samurai can do that a Fighter (already considered one of the lowest tiered classes) cannot do better while simultaneously doing many other things better than the Samurai.
Most of the Far-East themed classes from the Complete series were this way. Shugenja had incredibly limited spell selection to the point that the player chose very little of his character's core abilities. The fact that they were Divine casters (and thus able to cast in armor) was negated by their lack of armor proficiency and by having the worst Base Attack Bonus in the game (for comparison, most Divine casters get the medium Base Attack and medium or heavy armor proficiency). Wu Jen had weaker casting than wizards, and their "Spell Secret" class feature left them Blessed with Suck as it gave them free metamagic feats at the cost of crippling RP restrictions (each one came with a "taboo" that shut off the character's spell casting for the day if violated. And the metamagic feats weren't even the good ones!), meaning it holds the dubious honor of being the only class where Prestiging out is the only way to avoid crippling drawbacks.
Good Bad Bugs: Most rules exploits have been hilariously exploited with purpose built characters. These range from the "unintended but not game altering" to the countless much worse ones. Some examples:
Because alcohol is a poison in the game's terms, an ability with the effect of "save against poison for bonuses" gets activated by booze.
Until 3rd Edition, the description for the Cube of Frost Resistance said that inside the 10-foot cube it creates, "The temperature is always 65 degrees Fahrenheit," and that the cube only degrades if exposed to damaging cold. So, you could use it to walk through an unlimited amount of molten lava unharmed.
Base Breaker: Monte Cook, one of the designers for 3E and 5E. While he helped save D&D from death with 3E, there are some who loathe him for refusing to admit he ever made mistakes, and even more hated him after his signature Creator's Pet, Wizards, were overpowered once more in the 5e playtest.
Broken Base: Just watch any discussion between fans of Xth edition and fans of X+1th edition (or, sometimes, Xth and X.5th).
The edition war between 3rd and 4th editions. While there were some changeover gripes between 2nd and 3rd, this edition war has reached Internet Backdraft levels, possibly simply due to the availability of the Internet and the increased popularity of the system. Also the setting issue, where assorted players have flame wars over pointless gripes such as whether Eberron's dungeon punk setting is bad or not.
And, more recently after WOTC released a new set of core books called "Essentials", there is a heavy flame war between people who like "pre-E" D&D and those who like "post-E". The former group tends to call this new set "4.5", the latter gets positively enraged at seeing this number.
Not just linear warriors, quadratic wizards, but more linear warriors, quadratic wizards, exponential clerics. Clerics have better spells than wizards at most levels, and clerics can pick the best wizard spells from 1-5, and he doesn't have to prepare them. Not to mention proficiency with any weapon and every armor.
The White Box (1974), the Holmes Edition (1978), the BECMI ("Basic, Expert, Companion, Masters, Wrath of the Immortals", 1983) Rules Compendium (1989), AD&D 1st Edition (1977), AD&D 2nd Edition (1989), D&D 3rd Edition (2000), D&D 3.5 Edition (2003), 4th Edition (2008) D&D 5th Edition (looking like 2014)
with extra supplements for almost anything, a monthly publication that provided extra rules and modules, and that this is a gaming hobby where most people internalise the identity of the game they love, it's very common to find people complaining about whole editions of the game they've never played, never read, or in some cases, never heard of outside of the complaints of others. In the information age, the most noticeable version of this is the reaction to 4th Edition's latest extra rules supplements, but it goes back further than that. Especially given how complex some rules supplements are, it's pretty much inevitable that people will disallow some based on no actual play experience. This can make it awfully awkward to deal with legitimate concerns about any particular part of the game, since there's a lot of backdraft over disliking things other people like.
Fandom Rivalry: Some fans of weapon-using classes (I.E. Fighter, Ranger, Rogue, Barbarian, Paladin and Monk) don't get along with some fans of magic-using classes (I.E. Wizard, Sorcerer, Cleric, Bard, Druid and Warlock). Mike Mearls and Monte Cook encourage this, with Mearls taking the side of the Martial classes and Cook taking the side of the Caster classes. Unfortunately, since Cook had a bit of an Auteur License compared to Mearls, this usually leads to Cook making any and all casters into Game BreakingCreator's Pets. Fortunately for Martial fans (and fans of balance in general), Cook left the 5E design team, but not before they could fully remove his trademark of overpowered Wizards.
Fanon Discontinuity: Every edition has inspired Fanon Discontinuity. There is still a very vocal 2nd Edition fanbase that despises the changes wrought in the transition to 3rd, and not a few 1st Edition holdouts who consider 2nd to be a bastardization, and a handful of hardcore grognards who think 1st Edition should never have supplanted "classic" D&D (called it 0E [Zero-E]). The 4th edition gets it the most, and most fans couldn't stand the new alignment system. Players naturally gravitate towards the things that make them more comfortable. Of course, it's not like the books stop working when a new edition comes out, and any really cemented group is going to have lots of house rules anyway, so it's natural that players will remain players, even when they stop buying the new material.
Informed Wrongness: The creation of undead is regularly noted as evil, but it never really detailed what is wrong with creating a non-sentient being through the direction of energy - it just states that anything using negative energy is automatically evil. It gets even worse when golems (which require the enslaving of a sapient being) never have their creation demonized.
Internet Backdraft: Just try to say anything against or in favor of 4th Edition on a D&D forum, only do so if you have a death wish. Starting an argument against or for a particular setting is also ill-advised.
Paranoia Fuel: So wait, most (if not all) of the stars are Eldritch Abominations that want to eat us? And some of them can create avatars of their powers called Star Spawn?
They Changed It, Now It Sucks: It has happened for every edition change. Some might argue that 4E most of all, but veteran D&D players would point out that it only seems that way due to the much wider availability of the Internet. Interesting to note is that this is now happening WITHIN 4th Edition itself. Wizards has started to release errata/updates every few months, usually to stop overpowered exploits (although sometimes for other purposes too). Naturally, people have either declared it to be the best thing since sliced bread, or threatened to stop playing D&D. And with a new edition in the works, expect to see this yet again!
Unfortunate Implications: You have your elves. They're Usually Chaotic Good. Then you have your Drow. They're evil and black and the women are in charge. Always Chaotic Evil races in general fall straight into this, so let's just say that's just the start, but to be fair, the good drows are also a matriarchal society.
What Measure Is a Non-Badass?: 4th edition is very much geared towards heroic fantasy with the default assumption that the character is a badass, to the extent that it is mechanically difficult to create a character who is actually bad in an ability (as against 'average'). Likewise you can't really model an entirely green character who has picked up a sword for the first time as even a level 1 character can call upon fairly formidable powers.
Base Breaker: Uni, you either like her because she's cute or hate her due to her annoying voice.
Epileptic Trees: So many theories behind the reason why the series was canceled...possibly because the actual reason (ratings were dropping and the show was quite expensive) is disappointingly ordinary.
In Name Only: It was a decent cartoon, but aside from a general fantasy theme and a few borrowed classes/monsters, it wasn't much like the tabletop game. To name one difference, "casting" the Dungeon Master as a combination of Yoda and a Literal Genie undoubtedly led certain inexperienced game referees to produce a few terrible role-playing sessions. Of course, this same independence from the source material also gave the writers some much-needed creative freedom.
Ensemble Darkhorse: Jeremy Irons as Profion is the absolute best thing about the movie despite it's flaws. He is insanely so over the top that you are left in stunned appreciation for how a decent actor could go so deliciously over the top.
Damodar, who proved popular enough to appear in the sequel. Possibly because he kills Snails.
Xilus, leader of the Thieves Guild, most likely because Richard O'Brien makes him just so damn entertaining to watch.
Fridge Logic: The Emperess attack with a flock of Golden Dragons, a.k.a. Fire monsters. The wizards counterattack with... fireballs and Red Dragons!? So, you're attacking a creature immune to fire with fire!? WTF!?
To be fair, Red Dragons and Gold Dragons were only immune to fire from 2nd Edition onward; in 1st Edition, they merely gained +1 on their saving throws vs. fire. It was a little odd, though, that the Gold Dragons were never shown using their other breath weapon (poison gas, which even in 2nd Edition the Red Dragons would not have been immune to).
So Bad, It's Good: A lot of the movie is hilarious in its cheesiness, hamminess and dated effects (and genuinely hilarious on rare occasions), unless you're a serious D&D fan, in which case it's like being eaten feet-first by rats.