Well, you are right that a balance must be obtained. I usually do not criticize grammatical/mechanical/spelling errors if they are not excessive.
As a general criticism, avoid long strings of adjectives and long strings of "one line" dialogues. Those are the #1 mistakes of many inexperienced fanfiction writers.
When I write fanfiction, I always go over it again every couple months, expanding or condensing and adding detail to hone a "good" story into a "great" story. I encourage all to do this if you already don't.
RIFTED is right. If we want quality products, we have to ask them of the writers. And we should read the other fanfics. Hey, I'm guilty of reading and rating only AAML/AAMRN fics. But as RIFTED said, we should support and encourage all genres and their writers.