I would really like to see comments I have upvoted
I have used two strategies to go back to comments I have liked to read (whether or not I happened to upvote them when I read them). I either
a) use my brower's bookmarking capability to bookmark the exact link to the comment
or
b) I copy the exact link of the comment into a text file that I save for building FAQs.
I also make extensive use of HN Search and site-restricted Google search to find old comments on recurring topics here that are worth looking up again. Sometimes I find new good comments that way that I didn't see back when they were fresh. I try in general to upvote a lot, so the "saved stories" on my user profile here on HN includes hundreds upon hundreds of threads, way too many to be practical for going back to particular good comments.
It would be useful to be able to 'star' or 'favourite' a comment or submission, to have them accessible in a private list, and to see how many people had done this to your own comments/submissions.
I wouldn't like this to be generally visible though. As others have said, it creates a halo effect and discourages diversity of opinions.
I see no reason to use additional mechanism. Why not use upvote? I'm upvoting stories so I could see them later (as ColinWright noted), why not create a page I could see comments I liked?
Separation of concerns. You might want to be able to tag a post or comment as noteworthy but not want to upvote it. For example, to remind you to come back to it and add a comment explaining why its wrong or that you disagree with it.
Perhaps to address a limitation in browsers wherein it is not currently possible to scope bookmarks to a given site during a search for said bookmarks?
