Works really well with a controller, if you know what you're doing you can be pretty quick with one. Car handling feels nice with them with the way the controller is set up.
Overall improvments is generally the online mode, proper racing in official servers means generally no latency. Graphics are massivley improved too, almost all the photos i've taken show essentially what the graphics actually look like while racing. It's rather pretty.
And as I mentioned, livery editor is a welcome addition over old GT games. I'd say in some respects it's better than the one in Forza. All that is needed is some additions for better placement of items like a grid or some co-ordinates/rotation info.
I see GT Sport as more of a spinoff game like the GT Concept games, everyone knew what it would be and should have set their expectation accordingly.
Something else i'm seeing to is a rather large amount of people who didn't do basic research about the game then complain about it not having a career mode and such.