Guitar Hero 3 represents the golden age of the Guitar Hero franchise
The music in this game is interesting. There are over 80 tracks (with free DLC) and a majority of the tracks you will have heard before. There seems to be a big lean towards 70's and new music. Leaving out the two decades in between, my two personal favorite decades was a big mistake. Even with the lack of new wave, 80's/90's alternative, and grunge, there are only a few songs that I strongly disliked (go to hell AFI).
To answer the obvious, yes the songs in the game are tougher. Songs like Raining Blood and Through the Fire and Flames will frighten beginners, but with are beatable with practice.
This game features a co-op career, but it feels half-assed and cannot be completed through Xbox Live. The new Xbox Live features also feel tacked on. There is a lack of a decent lobby or party system. Plus, only two people can player together. The generic multiplayer is what brings this game down from being perfect.
Overall, the game has been made to be more playable. The game is more lenient towards when you hit the notes and the hammer-ons have been made super simple. Balance this out with the heightened difficulty and the game is just as hard as it was before.
With the gameplay as smooth as butter, I highly recommend giving this game a try. Guitar Hero 3 is one of the most iconic games of our generation and one of the best games too. It should also be mentioned that the achievements in this game are some of the most recognized and hardest. Experience this game before the Guitar Hero franchise is totally washed out.