Scott McCartney's Spet 10 online column has results of his informal survey of frequent fliers:

What Makes a Good Airline?

1. On-time performance

2. Fair pricing

3. Adequate seat room

4. Friendly, helpful employees

5. Frequent flights and direct service

6. Elite-level frequent flier perks

7. Assigned seats

8. Fast check-in/automation

9. Clean planes

10. Food


Clearly, Southwest understands these customers, having nailed the first 5 items. (Their on-time performance is worse these days, but frequency of flights tends to help out.)