I've been looking at options for a forum for ages but didn't find anything I liked 100%. However, the time has come to "just pick one already!" and so Rebrickable now has a Forum section :) The existing comments on Rebrickable are great for specific sets or parts, but lack the flexibility of a general discussion forum.
I created some initial forum topics, but of course these are subject to change. In fact, there's a Suggestions forum for anyone to suggest better ones!
To login to the forum, use the same email address and password you use for the main Rebrickable site. You will not be able to change your password in the forum (if I've done things right!), but you can change your display name and other profile settings. For now, it doesn't recognise when you are already logged into Rebrickable but I hope to get that working eventually.
Packaged forum software these days come with about a billion settings, so it may not be perfect yet. Over time I'm sure everything will be tweaked as required. This one is based on IP.Board which is the same as Eurobricks so hopefully most users will be familiar and comfortable with using it.
So please, I urge everyone to jump in and get the discussions started!