I'm unscreening your comment so everyone can see what we're talking about, I hope that's okay with you. :)
It is a fine line but I don't want this to turn into something negative with hurt feelings. The point is to celebrate and bask in the great fic that's out there. If you don't like something, it's fine to say that and to even say, "I would like to have maybe seen this: ", but the minute you bring in the author's thought process or how 'the author' could've fixed it, I think we're crossing that line.
I think a good test is: Would I be crushed if this were about me?