Immediate notifications dilema
Regarding the "... other forum software ..." near the end of your question: how about using the Flag module and its Rules integration?

Consider "somebody subscribing to some node" as the equivalent of "somebody flagging that node". And if your current technique to "send them notifications" is not flexible enough to do so in combination with the Rules and Flag module, then you may want to consider the Message module (which goes together with Rules and Flag very well also, and which is also used in some of the popular distributions).