Southwest has remained profitable for decades by not trying to do too much:
Simpler operations mean fewer things that can go awry and botch up the whole process. Consider, for instance, Southwest’s fleet of jets. While other airline fleets can employ 10 or more types of aircraft, Southwest uses just one, the Boeing 737. … Southwest also doesn’t assign seat numbers. Which means that if a plane is swapped out, and a new one’s brought in with a different seat configuration (even within the world of 737s, there can be some variations), there’s no need to adjust the entire seating arrangement and issue new boarding passes. Passengers simply board and sit where they like.
The Dish 