Well, I'm of the opinion that concrit should be acceptable as long as it is, indeed, constructive. I've seen some comments that made me wince because they just seemed overly critical, and there was no implication that the reader found anything to like. I can't understand leaving any feedback if that's the case.

I have to admit, it never even occurred to me to not offer any kind of concrit when commenting, because to me that's an inherent part of feedback. But thinking about them as gifts does put a different spin on that original perception. I guess mileage varies.