mobile_carrot:
I did twice this weekend and I'm feeling a bit guilty.
Don't! Unless, of course, you made the nasty comments in a nasty, derogatory way. I appreciate construction criticism (almost) as much as I do positive reinforcement.
If there's an aspect of a story I don't like, I'll find some way to say that while stil commenting positively on what I do like. If it's merely a matter of MY preferences, I'll often either remain quite about it, or star off by while something along the lines of "While this isn't my cup of tea," and then go on to say something positive. Sometimes, if the story was meant to seem more or less realistic, I'll make a distinction such as "Although this seems a bit extreme for real-life," and so forth. If I truly think something detracted from the story, I'll say that as well. It don't do this often, bbut I think it can be helpful.
I appreciate the same sort of candor from my readers. I may or may not agree with the commenter. If I don't I'll usually explain myself in a reply.