Oh, man, a bunch of things. But No. 1 on my list right now is what are known around here as arrogant roadie pricks, the kind of testosterone-driven, almost-always-young-male riders who terrorize the MUTs, make fun of everybody slower than they are, ridicule bikes they don't like and refuse to help a newbie broken down by the side of the road.
This isn't an anti-speed complaint. We've all been speed-obsessed at some point, and I probably still would be if I could still go fast. But we've had (just for instance) a rash of guys on the local multiuse trails hammering through the old folks and strollers, apparently deliberately brushing them back, with a bunch of minor crashes and one badly injured walker. One of the local senior citizen groups is threatening to start carrying canes to thrust into spokes.