Unlike the previous Forza games, you're no longer able to spend your way to winning. The new class system prevents you from entering cars that are ridiculously powerful compared to your opponents in the race.
Each of your cars is given a general performance number and a class, which takes every part of your car's performance characteristics into account - there's not only a class limit, but also a hard performance limit set on each race meaning you can't just mod your car to success. It makes the races altogether more challenging, which again goes some way to rewarding good driving.
On the subject of upgrading or modifying, the process has been made much more accessible for those who don't want to get into all of the nitty-gritty normally associated with realistic driving sims. If you're unsure which part of your car to upgrade, you can use the Quick Upgrade option instead of trawling through menu after menu of upgrades. This applies a balanced set of modifications that will take your car as close as possible to the class limit for the race you're entering, without going over of course.
The AI is one of the most important parts of a racing game - especially one that classes itself as a simulator - because it can make or break the experience. Turn 10 has done a reasonable job here, but it's frankly not the best in-game driving AI we've seen. Being barged off the track for being on the racing line happened far too often in our experience and, while the cars do back off in some instances, it doesn't happen as often as it does in PGR4, for example.
It's a difficult balance and one could argue that if the computer-controlled cars always gave way to you, the game would be too easy. There's surely a better way to handle this than just barging you out of the way every so often though?
Moreover, multiplayer games suffer from the same problem, only worse if our short forays online are anything to go by. They have been more akin to stock car racing than Formula 1 because there's no incentive to drive properly with damage turned off - you're no longer rewarded for smooth cornering and it's better to use your opponents as brakes into slower corners because you can approach them much quicker.
While it's difficult to solve the multiplayer problem, short of just finding a bunch of mates who aren't going to drive like idiots, Turn 10 has made the single player game less frustrating with the introduction of the Rewind feature. This debuted in Race Driver: GRID and while some argued that it made GRID easier, it didn't in our opinion, and actually ended up making the game less frustrating. The same is true with Forza 3 and the frustration of being unfairly barged off the track is dampened somewhat by its inclusion.
Overall, we've been impressed with Forza Motorsport 3 - it has built on Forza 2's strengths and extinguished many of its weaknesses. The AI is still our biggest concern, but Rewind does help to remove a lot of our frustrations with it. I still wait patiently for Gran Turismo 5 like I'm sure many PlayStation 3 devotees are (I don’t own a PS3 and won't until GT5 comes out), but Forza 3 has set the bar pretty high and you can play it today.