Nobody's mentioned yet a brand new Business Select (who chose a name with the initials BS?) benefit. Look at
http://www.southwest.com/fares/pop_fareCompare.html to read a comparison chart with the advantages of BS seat pricing - the third item is something everybody has been asking for for a long time:
Same-Day Flight Changes:
On your original date of travel you are free to fly standby or make confirmed changes (if seats are available) for another flight to your destination at no additional cost.