This should be rather informative.

Taken from redbrunja :

Okay, flist, time to get constructive and critical... but nicely, please.

Pick one piece of fanfiction I've written.  I'll tell you one thing I didn't like about it, one thing I did and one thing I wish I would have done differently.  Then, you give me one thing you didn't like, one thing you liked and one comment about it.  The comment can be a question or in general remark or whatever you want.  Post this in your journal so I can do the same.