My first two:
Watching a thread.
I have never ever heard of a need for daily digests or whatever. For me it's always a question of with or without e-mail notifications. And that's how it's done here.
Watch this thread -> check
and receive email notifications -> check.
Simple and great.
Another thing I really like a lot is the textarea in the reply form that grows along with the size of your text...
I also think that it's good to give positive feedback when it's deserved. So go ahead, post yours.
Even when clicking "watch this topic" after you created it (in postbit), it doesn't reload the page, just uses AJAX. PERFECT!