I'm with you -- I don't post unsolicited concrit at all. If I read a fic and like it enough to read to the end, I always make a comment. I point out what I liked, and I usually leave it at that. If it's someone I know fairly well, I'll sometimes point out a part that confused me. If I see a typo, I'll point that out (and appreciate it when people do the same for me).

But I guess criticizing a gift someone created for someone else just seems a bit rude to me, honestly. For example, I wouldn't criticize what my sister got my mom for Christmas, no matter how atrocious I thought it was. If my sister asked for my opinion on her choice of gift, I would give it, but unsolicited? That just doesn't feel right to me. :-P