There's not a whole lot to be said about Lego Rock Band, to be honest. It's pretty much the same old Rock Band game, just with a Lego skin. Rectangles, now in the shape of Lego bricks, slide down the tracks, and you use the same plastic instruments to match the colors.
There are a few new features worthy of note. There is a new "super easy" mode, where a player only needs to strum or hit a drum to "score" (i.e., it doesn't matter if you hit the right color, so long as you hit something). It's very helpful for kids, spouses, parents, etc. who want to play along but just can't get it. There's also an "auto kick-pedal" option just for the drums, which could work for someone like me who just can't get two hands and a foot to coordinate.
Also, in the "story" mode, you have the chance to play challenges, where you use "the power of rock" to accomplish some task (demolish a building, summon rain for a farmer's crops, beat back a giant octopus). It's a little entertaining, in that it adds a bit of purpose to the game — except it is really only good for spectators. You still have to concentrate on the note tracks, which makes it difficult to appreciate the visuals going on in the background. (Seeing the roadies pop up to shout "Ghostbusters!" in the chorus is cute, but I've only been able to see it out of the corner of my eye.) Supposedly, if you're playing multiplayer, the challenges work by feeding notes to one player at a time, though; unfortunately, I haven't had a chance to experience this first-hand yet.
The game also lets you play "short versions" of songs (at least, the ones on disc), which is helpful when you're playing the same songs over and over again. (Yes, the song list, while including a lot of good songs, is fairly short.)
Lego Rock Band is mostly cross-compatible with the Rock Band catalog, which helps to make it less repetitive. Harmonix does apply a certain rating to the songs, though, and only songs that are rated "age-appropriate" are allowable in Lego Rock Band. I don't begrudge them trying to implement this restriction in their "cute" edition, but as my wife and I went through the Rock Band store and noted which songs were marked available for Lego Rock Band and which weren't, we found some rather surprising choices.
My older son used to be more into the Rock Band series (although his interest had waned in recent months), and my second son loves the Lego games. Also, of the songs on my MP3 player, both of them had really taken to the Ghostbusters theme song, begging for me to convert it for playback on their DSi's and listening to it repeatedly. I had thought Lego Rock Band would be the perfect storm then, mixing Lego and Rock Band, and including Ghostbusters in its track list. Plus, new features like the "super easy" mode would make it possible for my less coordinated younger son to play without getting frustrated.
Unfortunately, they just haven't been interested. Whether they've just tired of the plastic guitar genre, or they're just too interested in the DSi games they got for Christmas, I'm not sure. Even when they do put their Nintendos down and play the 360, though, Lego Rock Band just hasn't been their game of choice.
I don't regret the purchase, even if I am a little disappointed by the lack of enthusiasm I got from my kids.