The airlines have been very adept at price discrimination. They get passengers to pay more for first-class seating and baggage checks, not to mention the endless tinkering with ticket prices based on when you're flying and how far ahead you buy your tickets.
Southwest Airlines has historically been a different bird. Only a few years ago did it offer business-class seats (for nearly twice the price on some routes), which include priority boarding and a free adult beverage; before that, it was first come, first serve. It doesn't charged for checked bags, as its commercials celebrate. Boarding is based on checkin time, and customers don't have assigned seats.
Recently, however, it has introduced an EarlyBird Check-in program. For $10, customers can get an earlier boarding slot without having to win the race to check in online. The only benefits: less time waiting in line (though more time waiting in the plane for everyone else) and increased access to overhead bins (though this is not as much a concern as it is on other airlines, because customers can check bags for free).
Southwest has prided itself on treating everyone like equals (or like cattle, as its competitors would argue), but its continual move toward price discrimination is probably good news for both its most price-sensitive customers and its shareholders.