Would'n it be niece if in the personal nodelet you would have a sign idicating if there was a new comment (or a change to an old comment) in the thread of given node? It could be a simple '+' sing or something - nothing very intrusive.
This way we could use the threads in a manner simmilar to mailing lists. People could subscribe to threads and be noticed when there is something new in them.
As a side effect there would be more reusal of the information contained in PM, for now there is little sens in posting to an old thread - nobody would notice.
There was something simmilar proposed in 'My Active Nodes' - idea, but mine is a little bit more general. Now the Personal Nodelet is just another bookmark mechanism - actually a bit redundant since there is already one in the browser, this would change it to something new.