Rock Band 3's intro, compared to those of other music games:


This is Rock Band 3's intro. First thing to notice: you're not immediately assaulted by the opening chords of Cheap Trick's "Hello There", but instead some ambient street noise that takes a moment to segue into The Doors' "Break On Through". Meaning if you just want to get to the intro menu, you don't have to furiously hit A to try and avoid the sharp volume spike. Also, kudos for having mellow instrumental tracks in the background of the main menu; it complements the atmosphere set by the intro, and is far less annoying than hearing "Bad Reputation" over and over and over and over while waiting for the Music Store to load.
(Not to say Hello There or Bad Reputation are bad songs, of course! But when I'm starting up the game a few times a week for two years straight to play random DLC...) 
Second thing: Though it's obvious that it's live-action, the fact that they chose to go with it here implies a thing or two about the way the game perceives itself. Previous Rock Band games had intros that were based on metaphors (your band is like a car that upgrades itself as you make your journey); this one illustrates a rainy night, graffiti, running, streetlights, and rooftops. Its intent seems to be to capture the atmosphere of what it's like to play music late at night -- and the fact that it stars, by all accounts, normal people, makes it easier for the player to insert him or herself into the band-member role.
Def Jam Rapstar's intro takes a slightly different tack; it presents music videos from groups like Run-DMC, Snoop Dogg, and the Wu-Tang Clan pasted onto skyscrapers.
Though more abstract than Rock Band 3's intro, this is totally acceptable; it illustrates that these are all artists that have had their contributed significantly to hip-hop culture, causing it to hit the mainstream in a major way; it also highlights that these artists are some of the biggest names in modern music. The tagline, "'Cause we're taking over one city at a time", draws the player into the game's motif -- which works wonderfully alongside the mechanic of uploading your own original interpretations/freestyles in video form to DJR's user video database. "We're" taking over, and you can be a part of it! Don't be scared, jump in and join us! 
Compare these two examples to the way Activision music games have handled their intros. The original DJ Hero, for instance, shows off a series of CG-rendered Gorillaz knockoffs exploding a giant evil monolithic mechanical record needle for the benefit of a weird postapocalyptic highway rave. Grandmaster Flash and a glowy-eyed DJ Shadow make appearances.
Though it can be interpreted as a celebration of DJ culture, instead it feels more like an original animated feature with little to do with the actual act of turntabling. DJ Hero is a great, great game, but the intro makes it look like its producers want you to turn on the game and go "All right, I'm putting on my DJ hat! Spinnin' rekkids, takin' names! This is Flava Flav! Fix up, look sharp! YEEEEEAAAAAAH, BOYEEEEE!" -- caricaturizing the game's core activity and integrating the player into it only for as long as the game is on. In doing so, it presents a face of DJ culture that's so far removed from the real thing it's laughable. Which is fine; these are videogames and not tutorials (excepting Rock Band 3), after all, but this one illustrates a resoundingly different design philosophy from Harmonix's recent foray into reverence. DJ Hero's wacky dancehall image persists through the ingame menus, selectable characters, and other aesthetics.
DJ Hero 2's intro fares better, though only in the sense that it doesn't attempt anything like this. Crazy CG graphics lead from the Activision/FreestyleGames logos into the game's logo, with hip mixed music playing in the background. Nothing spectacular, but at least it seems like it's taking itself a bit more seriously this time.
However, I think the most stellar example of the way Activision's music games have presented themselves with weird caricatures is the intro for Guitar Hero: World Tour: 
Which not only assumes you're willing to take the role of a heavily stylized, stiffly-animated cartoon band -- playing alongside such luminaries as Johnny Napalm and Lars Umlaut -- but also presents the impression, I guess, that clarinet music is heavily sponsored by The Man and that rock-n'-roll symbolizes the spirit of rebellious youth. Never mind that Activision's choices for band-centric games have all been bands that are made up of old fogies by now, of course, or that Guitar Hero is a multimillion dollar product that required the approval of executives and shareholders to get made; it's time to tear up some hotel rooms with hands locked in devil-horn positions, and any other form of music can go piss up a rope! Eat your brussels sprouts, grown-ups -- us kids have our plastic guitars, and are ready to rock! Hell yeah, I'm gonna press start! Where's the Dethklok song on this one?

Guitar Hero 5 and 6 don't have intros nearly as spectacular as this; they're actually depressingly short in comparison. Maybe it's a good thing; these cartoons look pretty cheap. 
I cannot wait to see how this facet of these games evolves in the forthcoming years. Will DJ Hero 3 or Guitar Hero 2011 (named simply "Guitar Hero") present themselves differently as a result of Rock Band 3's drastically different approach? I certainly hope so; these are fun to watch!