Good points! I also find it difficult to balance niceness with usefulness in textual comments.

One thing that may be on some folks' mind is that expressions of appreciation that don't also add something empirical or logical to the discussion are not likely to themselves be appreciated. If you post something I appreciate, and I comment to say merely "I'm glad you posted that!" I would expect that hardly anybody but you would be glad that I posted that.

I suppose that I could send a private message instead, but I would feel a little bit creepy sending a private message of appreciation to someone I don't know. I think I'd be more reluctant to send one to someone I thought of as a woman than someone I thought of as a man, too. (I don't endorse that behavior, but I suspect I have it.)

I wonder if the existence of voting as a way of expressing "mere" approval or disapproval disproportionately affects expressions of approval. Downvoting as an expression of mere disagreement is somewhat frowned upon; so do people upvote to agree and comment to disagree?