People trailing wheeled bags behind them who don't have back-up lights and an alarm.

People who stop in the aisle to have a conversation and won't move until you practically yell EXCUSE ME in their ear or poke them.

People who poke you to get past you without even trying to ask you first.

People who spend the whole flight crouching in the aisle talking to their seated friend so that everyone needing to get past to get to the lavatory has to be an acrobat.

People wearing enormous backpacks who manage to get absolutely each passenger in an aisle seat as they work their way to the last row.