writing, thinking about dark fiction

I had an interesting Twitter discussion on dark fantasy versus "dark and gritty" fantasy earlier this week. I'm curious if you have any thoughts on dark fantasy. Personally, I always saw dark fantasy as being fiction that resonated much like fairytales and think of authors such as Tanith Lee. Yet I now see the term "dark" bantered around to define anything that isn't heroic fantasy.

What about you? What kind of fiction do you consider dark fantasy? And for a bonus round, what turns you off or turns you on about dark fantasy?