I can't really compare too well anymore. That time I flew anything other than Southwest was American in 2009 (BWI-DFW every month), and a couple of times on United in 2009 and 2010 BWI-SFO and BWI - ORD. These were mostly for work, and I think I got some special treatment flying on government contract fares -- checked baggage was included and I got in the early boarding groups. This was, of course, coach, and essentially no meal service, even the 5-6 hour transcontinental flights. The United curbside check-in at SFO was a real clusterf*, but otherwise, everything was OK and the flights uneventful. I had one flight north from DFW where we had some tailwind and were doing a ground speed of over 700 mph and got into BWI a half hour early.
Since then, every flight I've taken (except for my trip to Beijing) has been on Southwest. I have no real complaints, nothing special, but they seems to generally do the job OK. I appreciate the fact that checked baggage is included in the ticket I've used carry-on suitcases, and it's a pain in the neck. The open seating thing is a bit of a nuisance, and I generally shell out for "early-bird check in" so that I can ensure that I'll be in an relatively early boarding group. It's also nice that I don't have to run to a device exactly 24 hours before the fight to print a boarding pass.