This can be good or bad, depending on your perspective. I am used to riding the bus, so Southwest is first class compared to Greyhound. Southwest does not serve as many airports as other major airlines do. For example, normally I would fly out of Syracuse, NY, but in New York State, Southwest only serves Albany and Buffalo. They do not serve any cities in Montana, South Dakota, North Dakota, Colorado, Maine, Vermont, Pennsylvania, West Virginia, Georgia, South Carolina, or Iowa.
Price is definitely a major reason for SWA'a popularity.
You can save a lot of money with SWA's many internet-only discounts. The website also features weekly Click N' Save special fares. You can sign up for e-mail updates at the website to stay informed. If you fly often, consider joining the Rapid Rewards program. You can register for the program either online or by phone.