Overall, Rock Band 3 adds some great features and enhancements, but there are a few things you should be aware of before purchasing (I bought on launch day, so I had to experience most of these first hand)

I'm not going to chronicle every feature's positive and negative

One nice feature is that you get pro drums with the DL, but unfortunately you don't get any enhancement to pro vocals or guitar, and no keyboard support.

You can use the Keyboard as a G/B or a G can be used to play the Key parts, so that is a nice feature

of course they cost you a buck or too per song, but its a great feature.

The online DLC store/import menu is a great feature, allowing you to buy any of the songs in the catalog and import songs from (at least

Some details just aren't that appealing (as in previous games, the notes 'explode' when played, but this time they do so in a cloud of cheesy-looking smoke)