I think the biggest problem is with oversized carry-on baggage. If everyone would take the time to be thoughtful of their fellow passengers and only use regulation sized carry-on and to take the time to put it in the overhead bin in a way that was not wasteful of space, the world would be a happier place.

BTW, having passengers carry on excess baggage is one of the reasons to be an elite frequent flyer - the early boarding is critical to ensuring having overhead space.