I guage who i nod to by quickly assessing if they're an 'enthusaist'. BOBs (blokes on bikes who dont appear to have any interest in their bike) don't get a nod because they have no affinity with you, they'll wonder what the hell you're nodding at. Thats a sweeping generalisation, but you cant stop and check every one, so you just make a quick assessment. Sometimes i'll get it wrong and appear haughty, but i counter that by nodding to just about anyone else from roadies, serious MTBers to old men and ladies on countryside rides.
Some will nod or say hello, some won't. Their loss, not mine. I'll stay cheery and welcoming and keep nodding.
And there you make a huge distinction...people on countryside rides tend to be 'enthusiastic' cyclists, but commuting in town is different. There's a whole swathe of people out there to whom the bike is just a mode of transport and commuting is done as a neccessity, not because they love bikes or have an affinity with you. Generally the only people i nod to when commuting are roadbike riders or perhaps guys on expensive bikes. Not out of any snobbery, believe me.
Just a point..not all motorcyclists nod either. A colleague was a serious motorcyclist. He HATED it when other guys waved or nodded. Who the hell are they, they don't know me, i don't know them, fcuk em. But then he's a very...'inward looking' fella, if you know what i mean.