Would be cool to be able to see all the posts a user upvoted. Also would be nice to have a bookmark feature.

My main concern with upvotes is privacy. If I do it I'll probably make it opt-in. Also, I rarely go to someone's twitter profile and see what they're liking.

I agree on bookmarking! Created a GH issue https://github.com/stackernews/stacker.news/issues/71

Apparently I was able to upvote my own post once.

Yep, when you exhaust your free posts/comments you'll automatically upvote all your own stuff.