People who sprawl out on the floor in the gate area with no consideration for those trying to maneuver around them. This past week I was on a very crowded (and delayed) flight out of BOS and nearly tripped over a young man who had made a sleeping area for himself (complete with backpack pillow) right smack in the middle of the passageway between the seating area and the gate. He remained there until the flight began boarding.

Same flight, it was impossible to maneuver around the seating area because people had planted themselves on the floor between rows of facing seats.