Obviously, I have not written anything yet so this shouldn't be of much interest to anybody but here are some thoughts I have on what I want out of fanfics (primarily Harry Potter based fics):

1. I strongly dislike Dumbledore bashing. Did the man lead a perfect life? No! But who has? He did the things that needed to be done. Let's say he tells Harry everything when he's eleven I think that would only lead to a horribly unbalanced Harry. As for the idea promoted in a lot of fics that he is constantly using legilimency on everybody, we have no canonical proof that this is the case, sure his eyes twinkle a lot and he knows a lot, these are not proof, perhaps he is simply very wise due to living a long, full life and his eyes are really watery. As for his manipulations, get over it! Everybody manipulates other people. I think I can honestly say that, given the knowledge that Dumbledore had as he lived his life, I would have acted pretty much the same way.

2. Another form of bashing that I dislike: Weasley bashing. Just can't stand it. Are they all the most interest characters? No. Are they all smart? No. Do they all truly care about Harry? Absolutely (except for Percy sometimes). Is Ron sometimes a stupid git? Yes, but I don't want him to always be a stupid git. Is Ginny the most well developed character in the canon? No, and the movies did a particularly poor job where she is concerned, but that gives writers plenty of opportunity to flesh her out in interesting and (hopefully) non one-dimensional ways. I guess that gets down to my main problem with "bashing" it generally relies on one dimensional characters, the bashed characters are always stupid or always manipulative etc. It just isn't particularly good writing.

3. I also don't really care much for stories where one of the main characters is an OC. It just seems to defeat the purpose of reading fanfiction for me. But I understand why others would like more variation in their characters.

I'm sure there are more thoughts to come...

Dumbledore's Man, through and through.