Few more things that catched my eye in this discussion.
1. "Professional mode" is, by the words of Kazunori Yamauchi himself, a mode that will simulate (new) physics (engine) of the car movement as accurate as it is possible with new hardware and is ment for players who want to experience real professional driving.
(Thanks for quoting the source: GamePlay Magazine Croatia, www.gameplay.hr) Describing difference between normal and "pro" mode is not something I can do yet but in shortly - pro mode was definitelly more difficult (challenging at least) than the normal and felt like the real deal (I've tried driving VW Golf on N1 to pull some comparisations with real life experience). Result? Far more swinging, more leaning, more weight shifting, more everything. More "playing" in sandbox than in normal mode when going over the edge.
2. Stands with seats with Logitech G25 wheels on it, had three pedals (from left to right - clutch, brake and gas) but clutch pedal was out of function / non-functional. Options menu couldn't be entered so the clutch support in demo remains a mistery for now even thou Kazunori Yamauchi in our interview stated that he would like to implement clutch in new GT games very much.
(Thanks for quoting the source: GamePlay Magazine Croatia, www.gameplay.hr)
3. G25 wheels had "rumble effect" on them but the force of Force Feedback seamed like on very low level. I personally guess that was for the safety reasons and/or easier manuverability of broader audience (public purposes).
4. Some cars (f.e. Nissan GTR Prototype) didn't have designed interior (cockpit) like the others but all cars will have it fully designed in final version of the game. Racing car Nissan Z350 MOTUL couldn't be selected for driving in demo. Depending on the cars (Japanesse/American/European/UK's) in-cockpit view puts the driver on the right or left side behind the wheel like in real life that gives whole new perception of the track when driving. Perspective of the track from cockpit view (personally my favourite in every game that gives that view option) in Gran Turismo 5: Prologue feels very good and natural. Unfortunatelly, movements of the driver under inertion forces (known as "G-force" also) that appears when braking/accelerating/taking turns wasn't noticable (maybe even fully non present) as I expected them to be in turning at high speeds, fast accelerating or hard breaking. Maybe it was because of the strongly secured driver's sitbelt?
5. Replay mode in GT5P demo is good as it ever been in GT series. "Triangle" button gives you on-push leveled zoom-in, "circle" turns off/on background music, "cross" changes car/stage view, SELECT changes car views (bumper/cockpit/windshield/3rd person) and "square" turns off/on dials/tachos. With directional buttons on D-pad you can change focus on "target car" and with (if I remember correctly) L1 you can look behind the car. There weren't any video-music synchro mode available in a demo, but the (in)famous transparent circle ring in the middle of the screen during replay was once again present. Why? I've never knew that.
6. When playing the demo in multiplayer, "game-pauses" (they definitelly weren't lags and I wouldn't called them frame-drops either) were noticable. Sometimes they've lasted for about 1 or 2 seconds and that wasn't nice experience. At least the car(s) stayed where you had him before the picture stop moving so you knew what to do when it continues. I guess that was happening when someone new joined the multiplayer game (respawned with car in pit line) and the time for "one session" driving in multiplayer was limited to few minutes (if I remember correctly - about 3 minutes or so - I must check my videos).
Keep it real, drive safe, live and let live!