I've met plenty of great people from both disciplines and plenty of jerks from both. I do however think that mountainbikers are more easy going and relaxed, riding for fun, while roadies are more serious and focused on training.
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"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger