For one thing, it's stupid. Yes, that happened on top of the Apple Corps building. Go art!
It's not even bad in a funny or noteworthy way. It's completely unremarkable. There's no buildup to speak of, nothing to denote that this is an ending and not just another one of the segue* scenes that bridge chapters. The opening cinematic of the game is more climatic than this; as a matter of fact, it contains most of the goingson of this ending in itself. There's not even a real final song, save for a song called The End (icwutudidthar) that comes after the credit roll(?).
I can't really dissect it further without trespassing into a discussion of pacing problems that the game as a whole has, so I'll come to a close with this: there has never been a more profound "are you kidding me, that's it?" moment that I have ever experienced while playing a video game than that of TBRB's ending. Be forewarned.
*EDIT: A bad ending is a bad ending no matter how you slice it. I remember the end of Guitar Hero 2, where Free Bird was essentially an ending in it's own right, and the words written in the stars were just a nice bonus. TBRB barely attempts anything like this and I'm surprised you're all so forgiving of that.
And a game doesn't have to have a plot to have an ending. Or at least, it doesn't have to have a culmination that's plot-centric in order for it to be compelling. I can think of a few instances of this (Punch-Out Wii, most recently). Again, TBRB doesn't even approach this.