They already have this, LH. Click on 'my replies' from the main forum page. It's right there under the welcome bar, next to 'my account, watchlist, and my posts.'
If you haven't posted though it won't come up there. Occasionally there's articles I'd like to comment on but not right now; usually I never find them again when I come back to write something.
I'd like to see this feature as well, although I can't think of a practical way to implement it. Chances are the watchlist is as close as we're going to get, although I agree with LH that the emailed updates are annoying.
My feature of choice would be better cross-platform compatibility. Firefox on the Mac doesn't post articles properly (it can't interpret carriage line breaks) and Safari can't even show the post comment box.
It makes using JU a bit of a pain sometimes, although as I don't post a lot of articles I get by fairly well with Firefox.
But if you're going to be introducing new features it'd be great if they could be tested to make sure they can be used by non-PC users.