I've noticed when a question is re-pushed to the top of the question stack (and thereby making it onto the home page of Stack Overflow) it can be tricky to figure out exactly what about that question (or one of its answers) has changed to warrant its re-push. It would be great if alongside "oldest", "newest", and "votes" answers could be sorted by "active", those that have had comments added, edits made, etc., going to the top of the pile. That way (for example) active threads of discussion about particular answers can receive fresh attention.
Well, on the homepage you can now click on through to the activity itself:
Additionally there is the (experimental) timeline view:
I decided to drop the "oldest" answers sort in favor of "active" answers sort. Since it's kind of redundant with "newest" (same thing, just in opposite order) whereas "active" is different in a way that is actually useful.
Make use of the not-so-easy-to-get-to-unless-you-hack-the-URL feature of the timeline view.
There you will see sorted top down the last activity made to the question as a whole. That includes comments and last edited answer revisions.
If Greasemonkey is your sort of thing, you can add that to your view set.
Activity is an ambiguous thing. When one person talks about edits, another person talks about comments.
Sorting by last edit time would be something both easily implemented and would probably be a nice addition to the site (when time allows).
But comments are a whole nother beast and something that no feature like this should be created for.
When a user clicks on links in either the Recent Activity or any of the tabs of his user profile page, the link takes the user to the answer link within a question that is the area of the user's input (the question or answer that the user provided or commented on).
When you reach the link, the post that was the target is highlighted and the highlight slowly fades. This could be a good idiom for what you are looking for. One idea would be to redirect the link on the homepage to the area that was updated. Though a great idea for those who loiter on the homepage, it could end up being confusing to the casual visitor.
Perhaps a good idea would be to maintain the fading highlight on the updated answer within the post for an extended period when coming from the homepage. This would allow those interested in what has changed, ie those who browse the homepage knowing that old posts are sometimes updated, to scroll and find the modification (assuming it wasn't a delete).