Wrath is an odd mix. It's unquestionably darker in tone than both classic WoW and Burning Crusade, but it's not hard to find funny bits and pieces scattered around. Därkseid of Deathwing, after having a disgusted murloc refuse to be rescued by him ("Ewwww, orc!") during a quest in the Borean Tundra, started a thread dedicated to peoples' favorite humorous moments so far in Wrath. All the normal world PvP events can be found here (aggroing a Storm Giant while being attacked and Feigning Death onto your enemy? Check!) but you'll also find a growing catalogue of memorable quest text and great NPC dialogue, like Akini's entry on this page.
For my money it's tough to exceed the tongue-in-cheek commentary you'll find in the books scattered around Acherus as you start a career as a Death Knight, including the one pictured above. Some of them are actually serious and give accounts of the Scourge's spying into the preparations made by the Horde and the Alliance for an assault on Northrend. Some of them, like The Death Knights of Acherus, are just hilarious, namely the entry concerning one "Harmony" whose period of service turned out to be extremely brief: "The name of this death knight was found in violation of common Scourge decency. Struck down by the Lich King." Well, Arthas, in such matters I defer to your excellent judgment.