Besides the 2 bags you get for free, the ability to rebook whenever you want and pay the difference / get a credit is so incredibly useful. I am not super well-versed on airline policies, but last time I looked, I think they were the only US airline that did this (at least for the economy class level of tickets).

One neat trick: you can rebook your exact same itinerary if you see it cheaper and get the difference as a credit (for the Wanna Get Away class fares; if you booked with the pricier ones, you'll get it back as a refund to your credit card).