I think this song has it all. I love when songs start out slow and melodic and gradually or suddenly get fast and heavy. This song does that when the chorus kicks in. But the chorus is heavy and not really that fast. So the tempo change isn't too drastic at first. And then it slows back down going back into the verse. With nearly every band I like there are songs that do this that I just love.
The thing about "One" is that after the second chorus it goes back into a slow breakdown of sorts at about 3:15. And that leads into the heavy chorus, which then leads into a heavy breakdown. So it's gradually building up to the 4:20 mark when Lars starts in with the double bass that ups the tempo a bit. Then at 4:38 the guitars come in to mirror the drums and drive the tempo and heaviness up even further. Then at about 5:22 we get into some good old Metallica thrashing. And of course that leads to the solo from Kirk which is one of his better ones.
Basically I like the journey the music is taking me on. Metallica and a lot of other bands are great at the pure thrash stuff that you can just bang your head along to through the whole song. And that stuff's great. But what I tend to gravitate towards are songs that can blend the fast, heavy stuff with changes in tone and tempo. And I think "One" is a perfect example of how to do it.