I have the same problem picking apart Bernstein as Beethoven--this is way more complex than, say, a Gwen Stefani song, and I just don't play on that level. So I'll do the best I can to explain why I like this song so much.
And that would've been enough, the cute lyrics and the nice melody, but it doesn't stop. It builds, from one voice, to two, to a duet with an increasingly prominent orchestra, to an entire choir.
Then the tempo picks up and the orchestra swells, and then all the instrumentals cut, all at once (around 3:06 in the video above), leaving the featured vocalists and chorus in kind of a Wile E. Coyote-just-before-free-fall moment.
It's difficult to describe, apart from being fucking awesome. It's like being able to see inside a huge machine, without all the supports or the exterior shell. It's just a complex, slightly unorthodox, very loud, completely all-out moment of vocal transcendence, which just hangs in the air without any orchestra to prop it up.
Which is why I'm not including this song this high on the list to be pretentious or sound sophisticated--you just can't do that shit in pop or rock and roll.