Quote Originally Posted by CGMidlander View Post
-Official forums may have moderation rules/policies that some players find disagreeable.
-Accessibility can be better on unofficial community sites. The official forums aren't super pleasant to use on a mobile device, for example.
-There is also no live chat client interface on the official forums, whereas unofficial sites may have their own VoIP servers or IRC chat.
-Sites like Reddit have features that the official forums do not. The live Nexus light bonus thread is one example.

Those are reasons just off the top of my head.
Don't forget the lovely 1000 character limit and daily post limit. -.-