This suggestion is simple enough... It would be nice to have the ability to comment on a user's page (or maybe it would be in its own comments tab/section I don't know.

Basically, it stems from the fact that I sometimes see cool activity from other users on the home page (recent additions, completions) but do not feel "right" just adding someone. I want to be able to tell them, hey that's cool that you also play these games, and so I'm friending you!

Or maybe a private messaging system (which I don't think exists???).

Ah yes, commenting :P This is definitely on my list and has been for a while. It's going to be included in the revamp of the Overview page...whenever I get there haha. I still haven't figured out what I want to do with that page, but I do know I want to add commenting and highlight your friends and what they're up to (feed / forum posts, now playing, recent completions, recent adds, etc).