One thing to keep in mind is that just because a forum is ostensibly about mountain biking they don't always serve the same purpose.
Rotorburn is a very free and open forum though it has got some limits such as with the discussion of illegal trail buidling. Threads on these topics will get "censored" to have a nice heated word in the pot. But basically they are just deemed inadvisable to discuss. Overall though pretty much everything gets talked about.
Some other websites are more closely linked to advocacy and are very heavily read by opponents of trail advocacy. Sometimes innocent comments that have been said on mtb forums get thrown back in the faces of trail advocates and cause lots of problems. These can be fair enough and honest comments and riders shouldn't be afraid to say them, but sometimes having really stupid, or inapproriate material out there is very harmful to the trail advocacy cause. This stuff is better removed and to be "censored" (yes lets use the heated word again) than to have as a problem down the track. Some of this material may not seem stupid or innapropriate to the poster but can make the mod with advocacy hat on squirm.
NobMob is very linked to advocacy and very heavily read/monitored by local council, NPWS, green groups etc. It is a site that therefore is a public face of mountain biking and has to have a responsible level of discussion even if at times this does take some freedoms away. (Same with the site listed in my sig.)
So if you want almost totally free discussion rely on rotorburn and don't go to NobMob. If you want to get what is useful from Nobmob, or other sites, play their game, get the good and ignore what you don't like.