My main gripe with all fanfiction is that it kind of saddens me to see people with real creative potential waste it on other people's characters. Some of the writers (occasionally) have true talent, and are capable of producing well written pieces, yet I can't help thinking about what they would be able to accomplish if they set out to write something they could genuinely call their own.
Saying that, I'm quite glad that most fanfiction writers don't write their own material, otherwise most things wouldn't even have a passable setting or developed characters - without stealing from a competent author, they'd all be pretty pathetic.
It's like this - if you put poison in a dog bowl, it doesn't matter whether your dog eats it or your neighbour's dog, 'cos you've still given it rabies.