I think the modality is different on DW and LJ-style sites because to comment, you usually go into someone's blog and talk to them with the understanding that they will see it as a direct comment towards them. Tumblr (well, my experience of it) was hard to have that personal connection bc it was just hard to track things.

That said, since I added the "want to leave a kudos" button, the only people who have used it have also commented, so it might be a self selecting option.