Hell exists in WoW, or the idea of it does at the very least. The word hell has been used in rather similar connotations to how we use it.
One of the Stormwind Investigators in Westfall for the new quest line there says 'what the hell?!' I believe, can't remember, did it last night at 3 am.
Only reason I'm making this post is I just saw someone say 'may he burn in Fel.'
... yeah... the little things annoy me.
As a side part... it could be argued that some of the Horde uses hell in the same context as us... at the very least it'd be the elves and orcs... maybe.