I don't really have time to go back through this whole thread, but around SE London/NW Kent it's become particularly unfriendly in the last few years. As often as not, it's the lone wannabe riders on one of five high-street brand bikes, invariably set up all wrong, mashing too big a gear for the terrain and their physique, eyes glued to their computers mentally w*nking themselves off over their bloody average speed. ("Ooh! 18.623546mph. Ooh! Ooh! Must go faster!! Must go faster!!") pretending they're in a race or visualising themselves in their next triantelope attempt. I find that most of the guys who race (or who have raced at some point) generally spin and pootle around at low/medium intensity - not feeling they have anything to prove and will happily acknowledge others. Road cycling seems to have attracted more than its' fair share of unfriendly poseurs in the last few years, and maybe one of the reasons that I find myself running more than cycling these days.
I'll keep acknowledging other riders around the quiet lanes when I'm out (the response ratio is 20 % at best), as there might be someone out there who's a bit lonely and has gone out for a ride, maybe hoping that somebody will acknowledge them - even just to validate their existence.