My idea depends on the commenting system, so apologies if this has been covered.
I know private messaging is being introduced. I guess my idea is applicable for profile commenting: a symbol to state if the comment had been replied to, as sometimes it's difficult to remember.
If someone comments on your profile, you get a private message, or any sort of thing happening with your profile you get a notification of it at the top. It'll be a red number showing how many notifications you have. Sort of like Facebook, when people comment on your profile or a comment of yours you get notified.