Yes we all know Gran Turismo is the best racing series ever and that's why we're dying to play it online! We almost got there in GT4 in the sense that now you can play over a local network with your friends, maybe GT5 will be the one.
Other than that biggie, well the graphics are still awesome, and lots of nice little enhancements here and there. You can transfer licenses and cash from your GT3 saved game which saves a lot of time.
The graphics are stunning though. Finishing a race is kinda just where the fun begins because you get to watching a replay and fully appreciate the work that was done on the scenic background your just raced through.