I loved the early Gran Turismo games, especially 2, 3 and 4. The challenging license tests, the wide range of cars, tracks and races, the gradual progression as you win more cars and unlock more features, etc.
GT5 was a let down with too much emphasis on B Spec (where you instruct an AI driver rather than drive yourself), too many lame cars and a very slow grind to progress through the game. The graphics were fine but I got sick of doing tough, long races only to win a Toyota Yaris or some other extremely weak car. And having to do B Spec, which is boring and frustrating since the AI drivers are so tame and clumsy, just to win certain cars. I never even got up to the endurance races because it would require too much repetition and waiting.
GT Sport improves the gameplay by not having B Spec at all. There are driving missions, license tests, online racing, tournaments and track time trials. To be fair, I haven't played online yet so I can't comment on it. The rest is fun but too easy overall. I usually get gold first go and sometimes beat gold by seconds which suggests the cut-offs should be tightened a bit. The graphics are great and the gameplay is well designed overall, with lots of options which are easy to change.
The biggest problem is the prize car mechanism. Firstly, finishing a tournament or series of races or endurance race doesn't win you a car. You just get money, mileage and experience points. That's annoying. Winning cars is part of the fun. Instead, if you drive about 42km in a day, you win a car. But it's random. So most days you win average cars, sometimes you win mediocre cars and rarely you win a good race car. But about 20 times I've won a car I already had. When I sell it to unclutter my garage, I don't even get money for it. It's a terrible design.
There isn't even much reward for completing races. Finishing all the arcade races gets you nothing, as I understand. The menus don't even show which races you've won or what percentage until you navigate into the race series which is time consuming.
It's good that most races are balanced, but there could still be more variety. It's too easy to buy super-powerful cars early on and many of the cars in Gr. 1, Gr. 3, etc., feel very similar. There's not much sense of progression, just random races to do which have little or no impact on what cars or money you end up with. Everything's around the same difficulty since the power is balanced and you can access everything you need near the start. It just takes time to save up and hope you win useful cars each day.
There are some cool features like changing the fuel consumption to maximise power or efficiency and seeing the breakdown of your lap times by segment. You can replay races and take photos, controlling the camera and applying effects. This is pretty cool but also distracts a little from what should be the main focus. The same goes for designing cars and clothes. What does it matter what colour the helmet is? I don't even need or want a virtual driver, only the car.
I'm looking forward to GT7 since it looks like they're bringing back a lot of classic tracks and hopefully going back to what made the early games in the series so fun - career mode, with a gradual sense of progress and lots of variety and challenges, not to mention lots of unique and interesting cars and tracks.