First off, let me say I really like the way IPVM handles forum conversations. It is SO much better, IMHO, than the stateless, unthreaded, repetitive quotation hell that most sites use.
One thing that would help one not miss a recent reply would be to highlight somehow, (tastefully), any new replies since one last loaded the page.
In short discussions this is not a problem, but in high velocity and multi-threaded ones it can be easy to overlook when there are multiple new posts at the end of multiple threads.
It's true the 'sort by most recent' helps, but still requires some scanning as well and removes the threading. You also have to know when to use it.
Just a thought and keep up the good work!