On silting and courtesy -- I've dived around (not *with*, so far) silt-spewing, lawnmower-style divers who tear across the bottom bicycle- or flutter-kicking their way into low vis for all. On no occasion did I ever see one turn around or seem to realize that he or she might be the cause of the near silt-out. My OW instructor approached my class with "No offense, but since it's your first time down there, you guys are going to be pretty bad silters," then proceeded to stress (in the water, on subsequent dives) the importance of good buoyancy and smart finning practices so as not to ruin our own dives and everyone else's.
On my last dive trip, my buddy and I found ourselves on a dive with a DM and one other diver, whom we quickly dubbed "Hot Dog." He was following the DM and my buddy and I were following him (my buddy being more or less directly behind him), and she kept having to brake abruptly on account of his deciding to stop on a dime and perform somersaults. He kicked her pretty hard during one of these maneuvers, causing her to drift up a bit and struggle with her buoyancy (she was on her sixth or seventh dive, hardly an expert). Back on the dive boat afterwards, he (unprompted) laughed at her and made a snide "having some trouble with your buoyancy there?" comment.
Granted, she was following a bit too close, but he didn't have to cop an attitude about an issue he helped cause. Divers like this just make me shake my head.