I find in both mtbing and road riding there are boring moments, and thrilling moments. Usually the thrills come out of what you put in, ie, try hard/go faster and the fun increases, mtb or road.
I find some people on road bikes do not acknowledge you (I tend to smile/say hi to everyone) if you look too 'pro' or vice versa.
I find some people on mtbs do not acknowledge you if you say hi and are part of a different niche to them. This seems to happen more often to me than road riding!!!
I often ride in Lycra offroad, fat/slow people in baggy shorts look down their noses at me. Doesn't bother me, I only see them for a short while. They can rarely keep up:)