Forza really does look good.
I will wait for more direct feed videos though. With the in-car view, that video doesn't look as good as the best GT5 has to offer... I'll post links -
http://www.youtube.com/watch?v=2yfPvrvhiOs&feature=related
http://www.youtube.com/watch?v=KPAR7XLKwEM
http://www.youtube.com/watch?v=6SmAwkSzurA
But then again, that's just one video.
Btw, people bringing up trees. This is a racing game, not a nature game. Focus on the cars and actual racing. When you're going 200mph down a straight, you're not looking at trees...