The -2BDs and -2S7s have slimline seats that are fairly comfortable but very cheap and have not held up well at all. They will probably have to be replaced before too long. The -231s still have TWA's seats which are of much higher quality but very flat and very hard. The 737s are supposed to get nice Recarros!
I work for Southwest, but the views expressed are my own and do not necessarily represent those of Southwest.
I've flown Airtran quite a bit this year. I have never noticed it. But I don't typically pay attention to that. I don't really expect seats on a plane to be nice unless I am in first class. I'll try to see if I notice next time.