I am sure a lot of users are visiting Stack Overflow regularly (and all of the sites from the Stack Exchange Network) to check whether a new comment/post/answer is posted as well as if there is a new upvote/achievement/etc.
Which is why I am proposing a built-in RSS generator to be implemented.
I am aware there are some third-party solutions (such as Is there a RSS to the 'Stack Exchange Inbox'? How about to Activity?) but my suggestion is this be implemented within the Stack Exchange system.