(Please excuse me if by chance you already have these features! I would be delighted.)
As a long-time forum-user, I have often been annoyed by the failure to have "default subscribe to posted threads." I.e., when I join a forum, it is because I want to post a message. But don't expect me to remember every forum I joined--or especially to revisit that forum looking for more replies to my threads. If there are more replies, I expect an email! Otherwise I may have relevant replies--but never visit that forum again. This is obviously not good for the forum-owner as well as the new member.
I see now--even with my own thread--Xenforo requires me to check "receive email notification." If I did not happen to notice that--you can not assume that I will even know if there are any replies. Not good for me or you.
Why are things like this? Probably for spam concerns. Some people get annoyed by too many messages and do not know how to turn it off--and press that "spam" button in their hotmail--and the consequences are serious. I resolve this problem by setting my forum default as a 'weekly' email, not 'instant' or 'daily.'
However... for this there is no excuse... I might even revisit a forum but never notice "replies" to my own threads! I personally have learned always to look under "subscribed threads." But I cannot expect newbies to know this.
Solution: "new thread replies" added within "new personal message" pop-ups.
With no concern for email spam accusations--it makes sense to include "browsed threads" as well as "watched threads" and "personal messages" in the same pop-up. And to check by default the three member options, "Browsed threads pop-up notice," and "Watched threads pop-up notice," and "Personal message pop-up notice."
This of course might be annoying to some people. So just place right on the pop-up some check boxes: Turn off popups for |_| browsed threads |_| watched threads |_| personal messages... |Submit|.
And of course... please at least set the default to pre-check "Watch this thread" every time someone posts a message! (I don't see any excuse for not doing so.)