It's got a lot of great parts. One of my favorite U2 songs too.
Bass- from Adam's opening spinning part right before the drums kick in, to the grove throughout, the bass line is very memorable and is the backbone of the song.
Drums- Larry's catchy beat is very reminiscent of Madonna's "Justify My Love" in a good way and it plays well with Adam's bass line. I also like the middle of the song where he does that nice roll right before "...cause I'm already gone, felt that way all along..."
Guitar - the super high riff is sonically piercing in a amazing, cosmic way. Nice chord strumming in the chorus after Bono sings "Goodbye." Sparse guitar throughout but great when it shows up. Signature Edge.
Piano! - one of the few U2 songs to use piano and just sounds so classic and a perfect fit when it comes in with Bono singing "Goodbye" in each chorus.
Vocals - Catchy "Goodbye" chorus. Nice melody throughout with mid range to the falsetto parts "Cause I'm already gone" and "sun, sun, sun".
And of course the best part in the song is the very emotional, unforgettable delivery of the line near the end "It's an emotional, Good night, I'll be up with the sun. Are you still holding on? I'm not coming down. Whoo-hoo. I'm not coming down. I'm not coming down."
Bono also gives a great, powerful delivery of those lines in the live Sarajevo version (after which Bono lost his voice for the rest of the concert):http://www.youtube.com/watch?v=SUVPOR6I_gk#t=11m45s