It seems to me that GT games in the past have been very un-personal. By that I mean, there is very little actual human contact in Gran Turismo mode. There are the AI drivers but you never see them. The only times you see an actual human are when you are doing a license test and there are the little group of testers that congratulate you on passing the test, the people walking around in the menu of GT5 TT, when you drive a convertible, you can see your driver. I kind of like it this way, because somehow it all seems less 'fake.' If there aren't any canned conversations to remind me its just a game, then I am not constantly reminded of the fact that my Race team manager only speaks 100 words of english (or whatever language you may play in).
Sports Games are pretty impressive with the commentators. I am most familiar with FIFA games, and Andy Gray's commentary is great, but it does tend to get repetitive, which I fear would happen in a racing game as well (as someone already suggested). I agree it could be cool if PD pulled it off, but I don't think it would be very GT.
(I have only ever played GT3, GT4, and GT5 TT, so if there is some other aspect in another game of which I am ignorant, please forgive me.)