It started as a simple post from a member of a Gran Turismo fansite, and it seems to have expanded into some very good reasons for the game. Here they are, all 25 reasons...

1. High Definition Awsome graphics.
2. High Quality Surround sound.
3. New exotics - ex. Ferrari's.
4. Dynamic weather during races.
5. A huge number of cars to drive.
6. A good array of real life tracks.
7. All our favorite 'original' tracks that we have come to love over the Gran Turismo series.
8. A challenging and educational Liscence test, which teaches you key driving techniques.
9. It's the real driving simulator.
10. 16 cars on road and 12 in multiplayer.
11. Compatibility with all wheels of PS2.
12. It's a must if your a GT fan.
13. The new Drift Mode.
14. Drift with more people than Xlink.
15. It's the best racing game for the PS3 console.
16. Full online functionality.
17. 4 views which includes that new "In car view".
18. Advanced physics engine.
    Downloadalbe cars and tracks in the future.
19. World ranking board to set your fastest times against.
20. An extremely accurate and well modeled Nurburgring.
21. A vast selection of modern Le Mans Prototype cars.
22. The ability to make your own club. (source: Joypad)
23. You can upgrade your car with many aftermarket parts.
24. You can tune your car with custom setups (ex. suspention, gear ratios, etc.).

Things to note are numbers 11 & 14. Will it actually be possible to plug your PS2 wheel into your PS3?