I/O Damage.
Mechanical damage would be very important, but for that I would like to see over-reving engines, you could only shift down if slow enough. Not that silly 2nd gear 200Km/h downshifting.
If would be nice to see some suspension damage, but... hey, I just was informed that Eiger Nordwand track is no more. There are some heavy failures in this game, and please don't flame me, I love it, just would expect some more realism.
As for drivers who like to drive, not to hit, you must be dammed good, cause when pushing it to the limit I usualy can't do 25 laps without falling of the track, and sometimes 5 to 5 lap hit is possible.
Also I would like to add this fantastic cover of GT5P pre-release, by our Forza Motorsport Neighbours:
http://www.forzacentral.com/forum/showthread.php?t=28282
We're all grown up, who doesn't like Forza plays whatever else, but doens't need to be agressive. Whoever likes GT5 plays it, PD doens't make us buy the game.
Damage would be fine, and PD implemented some good features from other games, now lets see how this turns out with updates.