I tend to write reviews for things I like, rather than dislike.
Critique is nice, but some bad fanfiction cliches still have their market, such as mary-sues, ukefication, etc, so it's kind of pointless to go and say they're doing it wrong when their readers obviously like that sort of thing.
Perhaps FF.net could do with something like DeviantArt
has where you can request critique if you want it. That way those who want to learn will be more likely to get what they want, and others will be more likely to be left alone.