Guitar Hero has been, and will always be, about the music. I could talk about the customizable character models, themed stages and other visual attributes, but the truth is you probably won't notice much - if any - of what's going on in the background.
Hands down, Warriors of Rock features the best song library to hit the series since Guitar Hero: Metallica. Gone are Top 40's Pop Hits and "Party Songs;" all are replaced with pure rock songs bursting with power chords and drum solos. The list is a nice mix of old and new. You've got Aerosmith, The Runaways and Jethro Tull, as well as Metallica, Megadeth and Pantera. There's also a nice mix of "newer" groups, such as Anberlin, Muse and Flyleaf.
This isn't to say there aren't a few duds. "Dud" might be too harsh, but then there's a handful of questionable inclusions like "Bleed it Out" (Linkin Park), "How You Remind Me" (Nickelback) and "Self Esteem" (Offspring). They aren't bad choices, but seemed a bit out of place compared to the rest. But hey, there's nothing that says I have to play them... right?
Also worthy of note, content from previous games will automatically import into the game.
What I really like about the list is it goes for "fun to play" over "well-known." There are some cases where the two collide, but then there are entries like Orianthi. Logic would say go for "According to You," but "Suffocated" - a much better guitar song (though really, any of her stuff would work) - is instead included.
I also want to mention the game's inventive use of Rush's "2112," a 20-minute concept song. I won't spoil it, but... great job guys. I want more of this stuff.