Mark > why can't Moodle forums send an email when there is a reply to a thread rather than the whole forum?
I totally agree that this is a standard functionality in internet forums and should be included in Moodle forums. However this has been discussed in the past and the general consensus among moodlers at the time seemed to be that such functionality would "go against the general Moodle socio-constructivist philosophy".
However, the topic has been revived recently; you may be interested in this thread: Moodle Forum that allows subscription by discussion thread and vote for MDL-1626.
ATB
Joseph