This thread was really interesting to read.
I do post constructive criticism sometimes, but am more likely to post unqualified praise. And there's a bunch of reasons for that, a lot of which other people have mentioned.
A big one for me is that I have only so much time to spend here. So, when time's short, posting a few quick compliments on whatever catches my eye first is the fastest way to keep myself in the habit of participating, not just lurking. And I'm pretty non-confrontational, so figuring out how to couch criticism constructively and thoughtfully enough that I feel okay giving it can feel like too much work if I'm not in the right mood.
Also, I'm a fairly new poster whose style isn't super-adventurous or super-refined, so I sometimes think critical comments I make may come across as clueless, especially if I don't have time or energy to explain my thinking in detail.
I'm more likely to say something constructive if it's a K/R or a specific question, as other people have mentioned. I'm also more likely if something jumps out at me as a fairly easy improvement or seems slightly off in a way that's fun to figure out, and less likely if something's just not my taste or I don't get it.
For my own posts, I sometimes forget to mention directly, but I definitely appreciate constructive feedback. I've certainly learned from comments I've gotten. Although, like others have said, I do sometimes add my own filters for comments that seem less relevant to me. Which may or may not include phrases like "crisp white shirt" or "just tuck and belt with jeans."