OP, i don't think you understand what 'dark' means, when referring to media (games, books, films, etc).
I think the word you're looking for is 'sleazy'.
WoW is extremely dark. The only reason people don't really notice it is, most people skip quest text/stories, and the humorous aspects/quests are remembered more. also the bright cartoony artwork tends to make people think it's more tame than it really is.
The lore behind naxx/icecrown/icc alone would make most peoples' skin crawl and stomachs turn if it were a darker setting or more realistic. if they rated games on story/lore alone, WoW would be at least rated M, possibly A.