1) No change fees
Southwest doesn't screw you when your plans get screwed. You might have to pay a higher fare but there are no fees. And if your new flight is cheaper they give you the difference as credit for a future flight.
This works for canceling also. If you booked a non-refundable ticket (which all but the most expensive ones are) and need to cancel instead of rebook, you get the full value of your ticket back as credit for a future flight. Those credits expire a year from when you bought your ticket.
2) No seat assignments
Some people hate it. I get that. But compared to trying to get a seat assignment on the legacies if you are not an elite status frequent flyer or are booking within a few weeks of departure, Southwest's no seat assignment system gives a lot of people a chance at decent seats that they wouldn’t have on other airlines.
3) Two checked bags for free, third one for only $25
After the industry's recent baggage fee increases, Southwest still comes out ahead of the pack by allowing all customers two free checked bags. And if you need to check a third bag it is a reasonable $25. Yes, Southwest used to let you check three for free, but at least they still let you check two instead of one like most other airlines.
4) Their website is easy and it works
They make it easy to book a flight. And that’s what you want. No complex options and choices. Just click Book A Flight, pick your city pair, your travel dates, the number of people and click Go.
5) They don’t outsource their call centers
If you need to call for help you don't have to deal with the madness of an outsourced foreign call center.
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I'm not saying Southwest is perfect. They aren't. They make big mistakes. But Southwest gives people without elite frequent flyer status a similar customer service experience that you have to fly 100,000 miles a year to get on some other airlines. While United is my first choice when flying in the USA, I would never wish them on anyone who isn't one of their frequent flyers. They treat their frequent flyers great but everyone else like crap. That's why Southwest gets my recommendation for the typical USA domestic flyer who is taking a couple flights per year.