As said by someone else, it is like an upscaled Polo outside. Inside, i would say it is different, rather than any better. If they launched the Mk6 as a new car, you'd probably say the same thing! I sat in an 2.0 TDI Bluemotion and it was nice for what a is middle of the road specced car.
The infotainment screen is larger, there is an electronic handbrake and more rear legroom. The boot is split level, but nothing 'must have'. I think the GTI and R will look good, purely because they will look similar to the outgoing cars. Supposedly there could be a Diesel R too...a model for the sake of it IMO!
Either way, with them coming in at 100kg lighter than the Mk6, the performance variants should be pretty rapid. Emissions are lower and fuel economy better too boot. Presume they will have 7 speed boxes too to further help things?
Only worry for the new platform is the new A3 (same car essentially) was beaten by the Mk6 Golf in its road test by Autocar!