I generally don't like fics that are main character vs. Team Blah... can get too cliched. Of course, there are exceptions, like when the plot swerves away from Team Blah with good subpots, and when there are really good characters, description and stuff like that.

I also don't like it when an average fic posts about a chapter a month. In the case of really good fics, where the chapters are actually memorable and stand out against all the other fic chapters, it's alright, but with an average fic, you can hardly remember the plot after a month.

Of course, there's all the stuff WFM mentioned.