You all know it if you play with sound on. If you don't play with sound on then this has nothing to do with you, so you can go read something else or see what you are missing.
First off let me say something about the mobs I hate killing more than anything.
Broken Draenei - The noises these things make when they die and while you are acting them makes me cringe.
Quilboar - Again killing these makes me want to punch kittens.
Harpies - After questing again in the Northern Barrens I can say that I have renewed my hatred for harpies.
These three without a doubt are horrible things to kill and make me regret having the ability to hear for that moment.
Now what about those mobs that are fun to kill?
Gnomes - not only how they die. Doing this jump and spin but the noise they make just makes me want to kill more and more. This would be why when I see a group of Alliance I go right for the gnomes just to hear that "ugwof" or whatever it is.
Gnomes for the longest time have been at the top of this list. There was nothing I ever wanted to kill more than a Gnome. That was until I heard the new sound for a goblin dying. Like really have you heard this? It is absolutely AMAZING. I really could kill these things all day long. Now not to sound like some sort of psychopath or anything, but when they scream for their life with their last ounce of strength it makes me smile. I am actually very jealous of Alliance players getting to kill Goblins all the time in Cataclysm. So you now have something I want!
If you haven't heard the noise you honestly have to go hear it. You won't be disappointed.