Guitar Hero III doesn't do much with the Guitar Hero formula, but why fix something that isn't broken?
First off, the tracklist is probably the best in any Guitar Hero series with classics like "Paint it Black," metal thrashers like "Reign in Blood" to fairly new tracks like "Miss Murder." There are tons of different tracks, and you'll find something in there that you'll love with out a doubt.
Next is the wireless Guitar, just buy it. It's great. It feels a lot more sturdy than the X-Plorer and thats because it is. The fact that it's wireless and has a detachable neck just makes it a lot more convenient of a guitar. Just buy it, you won't be unhappy with it.
Gameplay wise, the same for the most part. Notes come at you and you hit it. The two big additions to Guitar Hero III are online play which players have been asking for forever, and Battle Mode which... is sorta dumb. Some will love it, I personally don't really care for it. Its basically like Mario Kart. You hit notes that WOULD be star power but instead of getting star power you get a weapon, these weapons make your opponent get a broken string, flip them to a lefty or vis versa, ups their difficulty or a few other things. Like I said, some people will love it, but then again some won't really care for it.
Inside the game, a lot of the old characters are there but they took out a bunch that fans really loved. Gone are Eddie Knox (my personal favorite), Pandora (who has "merged" with Judy) and Clive Wilson. They have introduced a new character named Midori who is a Japanese Visual Kei guitarist. Quite honestly, she COULD be a dude, just look at some Visual Kei bands. One thing that is disappointing is that fact that there are no Japanese songs for Midori to play (Dir en Grey, Nightmare, L'arc en Ciel anybody?) Also you have the ability to play as Slash, Tom Morello, and some other fictional characters like a robot. Other little tweaks include a better organization of the song list and little things like that.
Overall, Guitar Hero III is just great. If you played the other two, buy Guitar Hero III, and if you've never played one before and want to get into it, buying the pack is a great start for you.