Currently when you are subscribed to a Forum or to an entry within that forum (topic or post), anytime someone else makes another comment or update, you receive an update. Neat. Unfortunately, on a busy thread, this can mean that you're getting updates a lot.
Having already checked with the support folks, I know that unfortunately ZD doesn't offer any sort of way to batch together all of those updates into a single emailing at the end of the day. A "digest" of activity on those subscribed forums/topics.
It certainly would make it nicer to see the digest of this, rather than continuous updates.
From what I understand, this can be accomplished via some clever API scripting - and I believe I've seen other ZD client sites accomplishing this, but we simply aren't able to place any development focus on that sort of thing right now, and we think it would be awesome if ZD offered this sort of thing directly within the core features.