For good reason. You'd think that after all the negative criticism GT5 got from their fans, PD would start focusing on some of the issues that people are most disappointed about, but they haven't. Sure, GT6 is an improvement over GT5, no doubt, but some very important aspects of the game are still lacking, such as sound, AI, and customization. I fail to see much, if any, improvement in these areas at all.
Oh, and the decision to not separate premiums and standards anymore is stupid in more than one way. Not only does it make the standards more implemented (which for someone like me, who despises them, is not a very good thing), but it also shows the ridiculous amount of duplicates among them.
On the bright side though, the physics feels greatly improved, the lighting is stunning, the UI looks good and feels much faster to navigate through, the tracks and premium cars are as gorgeous as ever, and overall it feels like a very decent game, but with too many flaws to make it great still.
I could agree with you in most of the points, but others not so much.
About the sounds, well no doubt it's the most importante aspect to improve in the game, if possible in GT6, it's something that adds a lot of personality to each car, if you get the right sounds you will enjoy even more the game, this eveyone agrees I believe.
About the AI, I think they tried to make more "achievable" (don't know if it's the right word) to new players, but they could make the game with difficulty choices (Easy, Normal, Hard) and this would fit to all kinds of players, wether they are pros or new to the series.
About costumization, well this is a bit "controversial" I think, I would like to have more tuning parts of course, but I don't need something like in NFS where you have millions of parts, just a bit more compared to what we have already, and the livery editor would be nice if it was a simple one, nothing too much complex, just the ability to put the brands stickers on the doors, tyre brands, etc., but even this would need licenses (nothing too hard I believe), other than that, just the ability to put some stripes, and a two tone option in some cars (like in a 458 Itália in real life you have the option to have the roof and the pillars in a different color), and for me would be enough, nothing too complicated like I said.
Well about the no separation standard/premium, is just a matter of opinion, you don't like it, but others like, so that doesn't really count to the overall quality of the game, because some people liked the ideia of buying the car they want anytime.