For me, there are some small but important improvements:
1) Speed of the UI. The actual loading of tracks/cars is pretty similar, but they've disguised it better with those transitions, and the main UI doesn't go that annoying 'GT' screen between each page. It has made the game far less painful to spend time with.
2) All cars in one list. GT5's used car section for the 'Standard' models was largely ignored by me, as it was far too annoying to wait for the car I wanted to roll around on their stupid rotation system. It basically prevented me from accessing most of the content. Now they're all available to browse through immediately, as it should be. The quality of the Standards is mixed, but I'm glad they are there, as I get to drive a bogey-green Volvo 240. I don't agree that they should be removed - there are some very unique cars, and I hope they can all be updated to at least the quality of the best ones, like the Ruf Yellowbird. I thought it couldn't be done, but they've done it... on some of them.
3) ABS 0 is now slightly usable on some cars. In the cars I've tried that do not use ABS in reality, for instance the F40, I'm able to run at ABS 0 and apply a good amount of brake pressure without locking up - about 75% of the pedal travel. In GT5 ABS 0 would cause virtually instantaneous lock ups, making it a totally pointless exercise. It has improved to the point where they can justify a brake upgrade, as some cars on racing slicks don't lock up on ABS 0 even at max pedal pressure. There are still a few cars that definitely need ABS on 1 though, like the Huayra.
4) Physics overall better, but not without some bizarre flaws.
5) Visuals improved - compromised in different areas, but less jarring overall.
The detail on the new cars for GT6 is even more crazy than the Premiums in GT5, but it only reveals itself in photomode where the tessellation kicks in as you zoom ever closer. You can actually see it smoothing out curves before your eyes - it's not like a LOD switch. Never seen that in a game before. Amazing! I can't wait to see what they do with PS4.