Have you ever played a game like Ragnarok Online or one of the other generic Asian games? Most of them have mobs spawning at a fast rate relatively close to the cities, and there is no locking game mechanism to prevent killstealing. Most hunting grounds have dozens of players on the screen at the same time, constantly getting into each others way and stealing each others kills. Rather annoying.
 

I'd like to argue that Kill-stealing is also possible in WoW, but not in the definition to which we have become accustomed.

Say you get a quest from Farmer Brown who wants you to kill 8 Hungry Wolves that are threatening his flock of sheep. So you head out into the field where another character, Mxyzptlk, is already busy killing the Hungry Wolves. Mxyzptlk is a couple of levels above you so he's having no trouble killing the wolves. Especially with the Power Shield he has, courtesy of his buddy Bizarro, which means Mxyzptlk just runs around the field aggroing all of the wolves and once he's gathered them up he AOEs them all to death.

Well that's ok, there are still a few wolves that Mxyzptlk missed, so you head towards the nearest one but Bizarro zaps it with his wand, Shields himself, Shields Mxyzptlk again, then zaps the Wolf to death.

So you head towards the next nearest wolf but Mxyzptlk is heading towards it as well, in fact he seems to be deliberately targeting the wolves closest to you first.

Not kill stealing in the truest sense of the word, but what else would you call it when a couple of characters prevent you from tagging mobs crucial to a quest?

I actually had this happen with the Lakeshore Whelps when a soloing Mage, in mid-fight, turned away from the Whelp he was fighting to tag the Whelp I was heading for, and then almost died as a result. Yeah, almost died. I was watching his health go down and ready & waiting with /lol but he survived by the skin of his teeth. I was still really a level or two too low to take on the Whelps anyway, so I muttered FU under my breath and flew back to Westfall.