Here are the basics:

• You'll be able to play the World Tour with your friends via XBL (finally).
• There are 80+ songs total.
  
• You can play your Rock Band 1 downloadable songs in Rock Band 2.
  

And here is a list of the tracks that have been leaked thus far:

• "Any Way You Want It" - Journey
• "You Oughta Know" - Alanis Morissette
• "Testify" - Rage Against The Machine
• "Spoonman" - Soundgarden
• "American Woman" - The Guess Who?
• "White Wedding" - Billy Idol
• "Aqua Lung" - Jethro Tull
• "Bad Reputation" - Joan Jett
• "One Step Closer" - Linkin Park
• "Peace Sells" - Megadeth
• "Man in the Box" - Alice in Chains
• "We Got The Beat" - The Go Go
• "Kids in America" - Kim Wilde
• "Ace of Spades" - Motorhead
• "Hello There" - Cheap Trick
• "Pinball Wizard" - The Who
• "Panic Attack" - Dream Theater
• "Chop Suey" - System of a Down
• "Give It Away" - Red Hot Chili Peppers
• "Everlong" - Foo Fighters