NamePros is a popular and active domain forum with over 100,000 registered users. The conversations on this site move at a VERY fast pace, so it will require a bit of time just to keep up.
Users have significantly different expertise levels, and it sometimes feels that expert posts are often lost in the landslide of questions and advice from novices. This makes it difficult to discern between the good advice and not so good advice but well worth participation.
When it comes to finding fast facts (ones that cant be skewed by opinions) and keeping up with news, this forum is top-notch. The forum also has fairly active buying and selling sections for users to negotiate their next domain name transaction.