Since the Brilliant redesign, voting on notes is no longer possible. My guess is that the voting has been removed because the notes only appear in the feed and not in a list where they can be sorted on the amount of votes each note got.
I think voting on notes should come back. Here's why:
Voting on notes is a great (and easy) way to show your appreciation.
If users can vote on feature requests, the staff can easily see how many users like the idea, helping them to decide if they should implement it or not.
It would allow the staff to reintroduce a notes-only page, where we can browse notes by tags, votes and how recently they've been posted.
Sometimes I come to Brilliant.org to solve problems, sometimes I come to see what the community is up to and to interact with them. Since the redesign, it's become a lot harder to do the latter.