A snap vote on features that people would like to see.
Upvote based on some sense of what the community should want, agreevote (the tick) to adjust for you personally.
There is a feature request thread, but I sense that old requests are getting lost. Hence, this.
Edit, I'm shifting my suggested voting rules. Agreevotes should now just be about the reasoning of the comment, as usual. They were basically entirely correlated. It was a trial and I was wrong.
Agreevote and disagreevote on posts. Sometimes I'd like to upvote a post without implying I agree with it.
I'd guess the reason this was done for comments first is that posts are much longer and more complicated, such that it's often not clear what "agreeing" with the post even means. I think it's plausibly a good feature for posts, but I think it makes a lot more sense for comments.