The core game is nothing more than Rock Band. Story Mode unlocks new venues by earning LEGO studs instead of money as well as earning fans and stars (based on the performance of each song played).
Each new venue is unlocked by buying a unique vehicle (that becomes more and more extravagant as your band gains popularity). In addition to the new vehicles for each venue, a specific challenge is unlocked as you earn more stars. The specific challenge then opens with the iconic Travelers Tales LEGO humor and character work, in which you either battle ghosts (aptly playing Ray Parker Jr's Ghostbusters) or recharge an alien ship's power source (playing Europe's Final Countdown) without the benefit of using the Overdrive boost. Each new set piece is a great creation to look at, unfortunately there aren't enough different audience member models or skins so after a few sets the rocking hordes of fans look very plastic and drone like (perhaps intentional given the source material--but compared to Rock Band 1 & 2 is very noticeable and someone disappointing).
When songs are completed, studs are earned which can then be spent on the band's clubhouse as upgrades. The clubhouse has a TV viewing area which allows you to re-watch unlocked cut scenes, a practice stage (for practicing of course), a music shop (which allows you to purchase better looking instruments--which unlock as songs are compeleted) as well as several other unique areas for hanging out doing what LEGO characters do best....act silly.
Along with buying new items for your clubhouse, you can hire members in your entourage. The entourage will boost fans and studs earned after each gig. The benefit of hiring all of the entourage members becomes apparent the further into the game. The tally screen at the end of each song takes longer and longer to count up the fans and studs earned as the bonus from each additional entourage member multiplies the number accordingly. The way that LEGO Rock Band implements the entourage makes a lot more sense than how hired help is utilized in Rock Band 1 and 2.
My biggest gripe and praise for LEGO Rock Band stem from the same thing. Song selection. If you were playing LEGO Rock Band without ever having played Rock Band 1 or 2 (or downloaded songs for those), then the song rotation in LRB becomes weak very quickly. However one of the greatest things about LRB is the fact that it recognizes most the library that you may have created by downloading songs for Rock Band 1 and 2 (or imported RB1 for RB2). This definitely gives LRB some additional milage.
As a music game LRB really shines for younger kids who want to play the game but may not have the co-ordination to play even on easy. Harmonix has added a Very Easy mode which to many veteran RB players would consider having all of the fun taken away, but for really young players who have taken a shine to the indomitable litte brick, this is a fantastic addition.
Fun for the whole family, LEGO Rock Band is a great game for trophy hunters as well. If you already have plastic instruments in the house (and kids as well), LEGO Rock Band is worth picking up.