So I finally had the opportunity to play Forza at a friend's house. I played it trying to compare it to Gran Turismo 4 and I'll try to give my opinion about it.
The graphics are good, though not as crisp as GT4's. Slightly more detail in the tracks, but the frame rate definitely was sacrificed to get this detail into the game. Of course I first tried the courses that are in both games, starting off with Laguna Seca. Feeling enthusiastic I set all the driving aids off with my first lap... well... that was a bad idea. Driving with a 500hp corvette without any aids is REALLY hard in forza. Harder than in GT4, that's for sure. Maybe if you'd put driving aids off and install normal tires it's harder in GT4, but the way I played both games, Forza is hard to handle without driving aids. The track looks pretty much similar to the one in GT4, with a couple of turns slightly easier to take (with driving aids on!).
Then after a couple of laps on Laguna Seca I went for a lap on Nurburgring, using a Tommy Kaira Impreza. It feels funny after driving many many laps in GT4 on this track to play in this game. The setting of Nur in Forza is more autumn-like, while in GT4 it looks more like it's spring. The track does feel different in a couple of spots but it's pretty accurate as well. Some elevations feel less steep, a couple of turns less sharp. One thing I did notice is that the buildings looked a little better than in the GT4 track, and at some points you can see spectators, which is cool. In Forza the track still has that Nurburgring spirit, yet some turns feel less distinctive than the way it was modelled in GT4, making it harder to see where you are at a certain point. What I did like is that I completely shattered my friend's personal record on Nurburgring when I went for a 2nd lap with that Chrysler supercar.


Tsukuba is the one I played next, pretty much the same as the GT4 one. Though it feels like the track was a little smaller with every turn or straight.
The new york track looks better in forza, the buildings feel like they're more in your face and tall. It feels more like an urban setting, though it still has those 90 degree turns and isn't much more fun that the GT4 course.
I played other tracks as well, which are pretty good. But I can't really compare those to GT4.
Now about the physics and handling. The steering is much more responsive and sensitive in Forza. It's also harder to lose control, but once you let go of the accellerator you get the control back. It's not like in GT4 where you spin out of control completely when you do lose control. Also hitting other cars feels more natural, like you actually hit a car, and not a complete block of concrete like it feels in GT4 when bumping into a car. It does make it easier to ram the lead car off track and take his position, enabling cheap tactics when not playing simulation mode. There doesn't really seem to be any weight transfer while racing. Overall I didn't like the feel of the cars as much as in GT4, but a lot of things in the actual races are better. Damage being one of them, now I've seen it in forza, that has to be in GT5.
One thing I did notice is that the game isn't as deep with cars. They car selection is definitely good, but the way their statistics are shown looks a bit cheap. Also the cars don't look as good as the ones in GT4, but of course they ARE customizable looks-wise.
Of course I didn't play it enough to really be able to compare every aspect of the game, but this is the overall idea I have now. Forza is a good game, it definitely has some things that GT4 should have had, but I don't think it's a better game. Though it IS the next best thing next to GT4. The lack of crappy old Japanese cars is a good thing though, they focus more on the supercars and in my opinion those are more important when trying to race.