1.) The graphics are definitely not as sharp as before. Say what you want about motion blur and lighting, but I can definitely tell that GT5 had better looking car models and environments. I think this is something that PD knew about, and this is why you don't have the option of looking at your cars in different lighting patterns anymore (in GT5, you could change it in the showroom).
2.) However, the frame rate, loading times, and number of things available on screen has definitely improved as well. I think that the races load two or three times as fast now.
3.) The UI is very well thought out and very easy to navigate. Since things are so much faster now, you can get from one thing to another much more quickly.
4.) The race layout is much cleaner and more intuitive. Before, the licenses felt really useless and now they've integrated them into the main campaign nicely. Recommended cars and having all vehicles under their respective showrooms (instead of that dumb used car section) are definitely helpful.
5.) There's still no qualifying in the campaign races. Wtf is this? Or is it something that appears in the later parts? IA maybe? If it doesn't, I'm taking a dump on the box.
6.) The sounds of the engines still need work. If I hit a car or a wall, there's this dumb glitch where the car just suddenly sounds like a vacuum cleaner, even though the gear didn't change.
7.) What's with the in-game music? It's like an homage to 80s rock and roll rejects.
8.) AI is still stupid, but hopefully it improves in higher difficulty races. I'm running cars well under the performance point limit, and I still haven't come close to being threatened to losing once.
All in all, GT6 is GT5. The differences are very very small, but it's definitely a better game.