To echo some of you and to add my own...
Fixie conversions.
The term "fixie"
Townie conversions
The term "townie"
Tires not lined up with the valve stems
QR skewers not lined properly.
Mustache bars
Poorly adjusted headlights that blind oncoming riders. While I am at it, headlights that are bright enough to light up a stadium for Monday Night Football.
People faster than me and people slower than me (to paraphrase George Carlin: everyone faster than me is a maniac, and everyone slower than me is an idiot.)
Dogs on retractable leashes. More accurately, owners of dogs on retractable leashes.