this stuff all sounds very familiar! i do a bit of everything so i see most of what you're talking about alot,
roadies, can be miserable depending on what i am riding/wearing, i find if your riding a good bike and wearing lycra they give the nod etc, if not they don't, for my nightshifts in the summer i ride my cross bike to work in baggy's (so i don't get the you look like a tit comments when i get to work) i remember seeing a local prominant shop owners son in full shop kit coming the other way, put my hand up and he ignored me but that maybe because like father like son etc!
mtb-ers, some good some bad, the downhillers are simply not interested in anything other than the people they ride with. i find the trail centre people aren't too bad at all, lost my wallet at afan, it was handed in with everything there, stopped and chatted with various people at various centres to ask about their bikes/kit etc, always had good chats. locally though i ride mostly on my own and when i see other riders most are ok
motorbike, i ride a crosser, most roadies don't nod, having said that, i don't nod at moped riders, mainly because they are spotty 16 yr old pillocks though!
point being with the motorbikes, is there is always a hierachy and people WILL judge you (if they don't know you) on what they see you riding/wearing to decide whether you are like them!
my 2p