If you have been happy with AirTran, why change??? Many people have no issues with AirTran. As long as you understand what your choices are and are good with that, then you'll be fine.
SW is a good airline. I fly both SW and JB. SW does now participate in the RAC program at almost all WDW resorts..which is very nice.
And yes, you do checkin at the 24 hr mark. BUT...that is not when you choose your seats. What happens is that, depending on how soon/quickly you get onto the checkin site, you will be issued a boarding pass group/number. They are broken down into the A, B, and C groups. Within those groups are numbers...groups of 5. You could get A15 or B42..it all depends on how many people beat you to the punch at the 24 hr mark. Then, as you board, you choose the seat(s) you want. There is no assigned seating..it's pretty much first come, first served. Those boarding further down the line (late Bs and the Cs) will have a harder time finding mulitple seats together.
SW now offers Early Bird checkin. For the price of $10 pp you can have SW check you in at the 36 hr mark..no need for you to do it. Then, at some point, after the 24 hr mark, you can go online and print out the boarding passes that SW got you. Those that have paid for EBCI will get better boarding groups than those who don't pay for it.
Now....with that in mind......those with any kind of medical issue, or minors flying on their own, are boarded first. Then, A1-15 people are boarded. Those are the fliers who have paid business select fares, basically fully refundable fares, which are sometimes double the web fares. Sometimes that grouping has no travelers. Then, those who have lined up in the A grouping, according to the numbers on the posts you will see at the gate area, will start boarding in groups of 5. Those who paid for EBCI will have the A group lower numbers, with the 'regular' fliers following them.
If you are a family, with a child 4 or under, you will be allowed to board in between the A and B groups. Now, this is a plus if you got a B or C group number.
if you have no way to pring out your boading pass, such as checkin in with a cell phone, you can just wait until you get to the airport and do it then.