there aren't really 'two' cultures, there are tons. i think i (and many others) span a number of these. here are some gross generalizations:
commuters - ride to get from here to there and back again, again and again, with stuff.
roadies - ride for fitness and fun and want to get faster (uphill)
mtbikers - ride for fun and excitement and to be out in the woods
fixers - ride for transportation and zen connection of pedal to road
track - ride for the thrill/challenge of a pure stripped down race
tourers - ride to see/be places at a bikey pace, with stuff - sometimes a lot of it
really, i'm not being fair to commuters and tourers - they're probably riding for fun too. cyclocross riders don't fit in any of these - they're just interested in self-abuse (and bike abuse).
back to the question though... "what makes mountain bikers different from road bikers?"
answer: the bikes they're riding.