Woody,
At your request I am posting 2 suggestions here:
1. It helps to have comments count at the top of threads. They inform on whether there were new comments since last time one looked at the threads.
2. Thread subscriptions triggered lots of email notifications for individual new comments. But even with those, it was difficult to distinguish in each visit old from new comments. Most helpful would be some option to get emails containing the new comments themselves– similar to what mailing lists provide. Then one could see if there’s anything personally interesting and/or something that one wants to respond to.