Actually some of the rest of the programming is also very good.
TNT is still broadcasting a number of programs from SD source material over their HD channel. They apply their own stretching algorithm to fill a 16:9 screen. It zooms the vertical a little and then aggressively stretches the sides of the horizontal. For the past couple of years this has been referred to on HD forums as TNT Stretch-0-Vision.
The live sports are almost all true HD.
Movies are 50-50. Some are excellent HD, others are stretched SD.
Syndicated TV shows are mostly SD, although some of them are native 16:9 SD, so at least they aren't stretched.
All in all, a very shoddy HD channel.