Din is a rather... unusual Redguard encountered north of Gnisis. He apparently lost his mind when he was bitten by a slaughterfish, and his fever is making him say very interesting things, like: "Muddled? Where? No! Don't play in the muddled. You'll get a dirty mind."
When wandering the countryside, you might run into a naked Nordic barbarian standing around. He claims a witch stole his clothes. Elsewhere, you might encounter another Nordic barbarian claiming a witch stole his clothes. And a third. There's also one in Tribunal, but he insists that he was only naked because of the heat, and complains that everybody thinks a witch stole his clothes.
When your character is fighting and someone hits them, normally they grunt or give a gasp of pain, but on occasion, they'll yell "Stoooopid!"
Crassius Curio. Besides the Squick he is simply so funny.
There's a spell called Emasculate that you can buy. It drains endurance. And there are a couple of bedrooms where you can find potions of Fortify Endurance under or behind the bed.
Dagoth Ur's reaction when you start attacking the Heart of Lorkhan.
The "Favors for Orcs" quest. It involves two orcs trying to hide from the guards for whatever reason, and you must play messenger boy between them. The first gives you a note to give to the other orc, saying that he's hiding "by the tree near a rock and another rock". He says that he's "good at hiding, so if you don't see me, that's where I am". When the quest is over, your journal notes that you "feel a bit stupider for ever having talked to either one of them".
Speaking of orcs, Ghorak Manor in Caldera has a few humorous areas. Creeper, the friendly Scamp with the 2nd most gold to barter in the vanilla game is on the 2nd level. Elsewhere, there is a female orc in a bedroom wearing armor up top... and nothing down below.
The note you have to deliver as proof that the Gateway ghost has finally disappeared.
If you bring up the subject "Telvanni Hortator" with Therana, she will tell you an endless story that goes nowhere. You have the option to listen politely, or cut her off and ask to be named Hortator. If you listen politely long enough, the dialogue option will change to "stubbornly continue to listen politely" and the dialogue text will display:
Mistress Therana continues to chatter amiably, with no sign of stopping. She looks like she could go on forever.
In the Bamz-Amschend ruins in the Tribunal expansion, you can discover a bed frame with two piles of ashes (all that remains of a pair of Dwemer after their entire race suddenly vanished) on the floor underneath. There is a metal tube under the bed, in between the ash piles, and a bottle of Dwemer Oil (listed as "p_dwemer_lubricant00" in the game files) off to the side.
The door to the room is also cracked, and has a pile of ashes on the other side.
If you taunt a particular Dremora to attack you, he'll say... this.
Dremora: After I kill you I'm going to rape your corpse. Don't worry, I'll be gentle.