Unfortunately it's not possible to put all cyclists into a specific 'greeting' category. Cyclists, like humans, are all different, are all subject to different moods, and have different priorities. Some like to wave and shout, some will casually raise their hand or fingers (in a wave, not in THAT way) just a fraction off the bars, some will nod sternly because they're seriously pleased to greet you, and others will stare blankly ahead and ignore you.
What you also have to understand is that not all cyclists are one... If you're a roadie then you can freely nod or wave at all other roadies and you will generally receive a greeting back. Likewise fellow MTB riders will always greet other MTB chums, but may often find themselves ignored by a roadie. It's like all those Minis who flash lights at each other, then wonder why the hell that Suzuki Carry was joining in.
Hybrid riders obviously fall down because of the very nature of their steed. They're neither one nor the other. They don't belong anywhere.
You can't force someone to wave back, in the same way that in a nightclub, without a healthy dose of Rohipnol you can't persuade fellow clubbers to join you on the dance floor for a chorus of the 'Hokey Cokey', or 'Superman'...
I, unfortunately am one of these needy people (who requires therapy) that Dragonwight talks about. I have a deep desire to belong to a special army of like-minded folks and get a warm fuzzy feeling every time one of my nods or waves gets a response from a fellow two-wheeled warrior. I pass the same people on my commute and I know exactly what response I will get from each of them. Most couldn't be happier to see me... we are kindred spirits who brave the elements all throughout the year. I'll wave, nod or smile at all riders, unless... they're a chav or a scally without a shirt on with a wilkinsons carrier bag tied over the saddle. I tend to ignore those ones.
There are a couple of commuters who I used to wave to who completely blanked me every time I tried to say hello. Needless to say I've stopped trying now and return their steely gaze without so much as a blink. If I ever met either of them in a nightclub I would most certainly not let them sit behind me if 'oops upside your head' was playing.