Comment: Some good posts here, but I'm surprised no one mentioned HORN HONKING:

People here (I've lived here since 1960 but this is a relatively new phenomenon due to the increased population density) honk their horns loud & long WAY too often.   In Manhattan drivers are now ticket-FINED $350 (yup) for unnecessary horn-honking, due to the urban high-rise nature of the city and how many people get disturbed by it.   Denver, with all the new dense growth, should start thinking about it as well.

Example:
    Hang out by the North Lake entrance to Washington park by Marion Pkwy sometime and listen to all the horns honking day & night as cars, bikes, and pedestrians try to deal with each other on East Virginia st going in & out of the park.   Tons of obscenities & bird flipping as well, with "A#$-v*le!!" being the most popular verbal exchange.

Rush hour late afternoon is the worst time, but it goes on all day & sometimes at night.  These honking jerks should be ticketed/fined just like in NYC.