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.
Queen Akorithi of Sentinel, like her late husband King Cameron, was a warmonger. Their designated-heir son was chronically ill and fairly uninterested in invading neighboring countries for their land, the latter of which was the kind of thing his parents liked doing when they could get away with it. Not only did they produce some more children and partially disinherited him, his interest in scholarly pursuits hit King Cameron's Berserk Button, so he and Akorithi murdered him by tossing him into a dungeon full of zombies and mummies just after a bad bout of the flu. By the end of Daggerfall, Queen Akorithi has used the power of the Numidium to conquer Bergama as well as the whole of northern Hammerfall. She dies of a plague not long afterwards.
Mannimarco, the King of Worms and the leader of the Order of the Black Worm, first appears in Daggerfall, then appears again as the Arc Villain of the Mages’ Guild quest line in Oblivion. An incredibly powerful necromancer and the world's first lich, Mannimarco’s study into the arts of entrapping and enslaving souls festered in him a deep hatred for peace and life. A sadist believing himself Above Good and Evil, Mannimarco sought out obtaining as much magical and political power as he could over the passing millennia. Sometimes this power was obtained via Faustian deals; in at least one instance he helped a princess arrange a political marriage in exchange for her firstborn child. After achieving godhood at the end of Daggerfall, a mortal part of Mannimarco was left behind, and decided to settle a petty grudge with a long-dead enemy by destroying the Mages’ Guild said enemy founded. Mannimarco became invested in creating black soul gems, which he used to steal the souls of those who opposed him, and plotted to capture the soul of Archmage Hannibal Traven to empower himself. The souls captured are aware while in the gems, and, once their power is used up, they are sent to a purgatory-like dimension called the Soul Cairn for all eternity. When a mage infiltrated his order, he had the mage tortured to death and then resurrected the mage as a zombie to be an example to the Guild. Mannimarco also took great pleasure in leading the attack on the Bruma Mages’ Guild chapter, slaughtering everyone present save for one mage whom he kept alive to tell tale of what happened in Bruma.
Demonic Spiders: Liches and Ancient Vampires. Unless you're resistant to their magical attacks, they can kill you instantly.
Game Breaker: If you're Fire Resistant enough, you can just walk to a lich and let him blow himself up.
Once you become strong enough, you can just camp out in the woods and wait for poor enemies to jump onto your sword. Or you can just make a big HALT train and kill it. Either way, you will have much better loot than you are supposed to.
Good Bad Bugs: There are a large number, but principle among them is The Void, the black area beyond the wall mesh in houses and dungeons. Walking/jumping/climbing against certain wall types at certain angles would allow you to pass through them, which usually resulted in you getting stuck or falling to the bottom floor, requiring a reload. However, with practice and luck, it was possible to use the Void to cut through the labyrinthine layouts (not to mention avoiding enemies) to quickly reach your quest objective, a practice usually referred to as Void Running/Walking/Surfing. Occasionally the Void would also swallow up enemies, which while helpful against Vampire Ancients at level 2 could render extermination missions unwinnable.
Of course the patch gave us Alt+F11, which took you back one step. It pulled you out of the Void, and with it you could also retreat out of the dungeon if you had forgotten to set the teleport anchor.
Much like Oblivion and beyond, stores have closing times. Unlike Oblivion and beyond, however, you do not get kicked out at closing time. This lets you take things without being considered stealing.