One of the great things about WoW is that they throw in those little things that make you laugh and smile. There are tons of things in game that have given me these reactions. Here are a few examples.
Druid Forms: Not so much the feral forms, but the Boomkin and Tree make me laugh. How can you not enjoy a giant lazer shooting owl with a fat ass that dances like Chris Farley? The tree is another of my favorites because they are so damn ugly. Not to mention their /cower animation is gold. The thing that I find silliest about them is their tree form casting animation. Especially the instant cast spells. Sometimes it looks like you are the only one in the crowd doing the wave.
Extreme Mouse Spinning: Obviously mouse turning is a good thing. What is extreme mouse turning? Making yourself spin in ridiculous circles. This one is mostly for my wife. She will literally laugh out loud every time I do it. It looks especially ridiculous on the Ram mount.
Training Mobs: Not training as in teaching and not mobs as in Moffia, but the act of pulling a ton a mobs at the same time...and running like hell. I used to do this so that my wife would not get knocked off her mount. This was yet another thing that gave her some good giggles. There can be some very comedic results when a Hunter tab targets Midknight in Karazhaz through the wall and sends his pet in. Now that is a train.
Pop Culture References: There are pop culture references everywhere in the game. Starwars, Startrek, Starship Troopers.....OK, so there are a lot of Star movies. One of my favorites are the Arathi Battle Master in Shattrath. The one who looks like He-Man and is named Adam Eternium. He even has his Battle Tiger along for the ride.
Eating Shitty Food: Literally. You eat nuts. Quest giver gets mad. Makes you shit out nuts....next person comes along and eats nuts. MMMMMMMM. Another one that made me laugh was the quest where you bring the rotten eggs back to Tobias the Filthgorger. He throw up and causes everyone else in the area to hurl uncontrollably. Definately one of my favorite quests for laugh factors.
To me, this is one of the things that seperates WoW from the competition. They are kind of like the Family Guy of MMOs. Tons of obscure references and shenanigans fitting together into a nice package.