Criticism of a gift on a gift comm is going to have to also be criticism of the good taste of the receiver

Are you talking about criticism of something the receiver requested? If so, yeah, that would be really poor form.

All I know is that I haven't left what I would call negative feedback--just made a couple of points, one concerning a slight canon deviation and the other about pacing. So I don't feel like I personally did anything wildly inappropriate. But I have re-thought my approach towards FB on these types of exchanges, not just because of this post, but also because the negative comments I've seen at the gift exchange I've been visiting have really turned me off.