Luggage and people who want to shove fistfuls of paper printouts at agents rather than OLCI or at least have information ready.

Overweight bags, oversized bags, too many bags, need for seat assignments together, special meals, and the like, all jam up the line.

As a practical matter, the average competent traveler ought to interact with a live human WN employee for about 2.5 seconds as he hands over or scans his BP at the gate.