Yeah bigger screen and higher resolution. Also LTE... so if you consider the things that people look for in a phone as screen size, processor, screen resolution, storage size, and network speed... then the AT&T HTC One X has larger screen, higher resolution, and faster network speed. So 3 of the 5 things people look for in new phones are a "generation" above the Tmobile HTC One S.
I don't really care about most of those things within reason which is why the HTC One S appeals to me. It's the right balance. That being said it's still lower end than the One X... even in the version AT&T is getting.
This is a huge Tmobile fail imo. Most of us will suck it up and buy it anyway, but like I said... that's not how to make a customer happy.