If I can't leave completely positive comments for an anon-gift fic, then
I try to avoid commenting on it at all.
IF, on the other hand, it was a fic written for me, then I would try to find a way to thank them for writing me a fic without necessarily touching on the things I don't like -- while I think that once you've put your work out in the world then any well-thought out criticism is appropriate, my mother taught me that you should never ever criticise a gift!