Polyphony was one of the best developers in terms of being able to wring as much power as they could out of the PS2, despite its difficult-to-program-for nature. I expect GT5 to be the same, a technical masterpiece that wrings more out of the PS3 hardware than we've seen to date. It should also be the most accurate driving simulation around.
That said, GT4 had gameplay issues too numerous to list here (and well-enough-known that listing them would be unnecessary). I'm hoping that Polyphony hired a Gameplay Engineer to make sure the driving experience isn't mired in these same kinds of problems. Again, not having played Forza, I can't draw a meaningful comparison in terms of the gameplay issues, but from reading user experiences and reviews, it seems Forza 2 was much better than GT4 in this area.
