At the moment, the only holiday challenge I've really had a chance to participate in is the sshg_exchange, but on the whole, when it comes to something like this, if I can't say something nice, I just don't say anything at all. The people participating in these challenges are (hopefully) giving their best effort (even if they are a pinch hitter) writing to a prompt that might not even suite them, so I don't want to be the person raining down on their parade telling them what I disliked about what they wrote or their style.