I have to echo the sentiments of those who consider this a very moving and powerful film.
The last act (if that's the correct term) of the movie, starting with when Clint and the kid learn of Ned's death is the most moving in my opinion. Watching the way in which Clint portrays his/William's old rageful nature surge to the surface while being informed of the details of Ned's demise, the cold efficiency with how he dispacthes the owner (can't remember the name) of Greely's, and how the viewer is finally treated to a glimpse at his gunfighting past during the gunfight are all spectacular.
My favorite scene, though, has to be when he's riding out of town after killing Little Bill and company. The way the expression on the faces of the group of prostitutes communicate the shock of people already accustomed to dealing with ugly and vicious men yet who've someone far more extreme than who they've been dealing with is truly masterful.
The cinematography is great, as is the music score. The emotions and thoughts evoked by the way the main characters wrestle with the internal moral battle concerning what they've set out to do, and the overall ambiguity of all the characters (as mcgredo mentioned) lend this movie far more ....gravitas (hehe)... than most other westerns.
A very excellent film, worthy of all the oscars it won. I own it on DVD, yet it is such a "heavy" movie to watch that I only get around to it when it happens spontaneously, such as saturday night on tv with nothing else worth watching on.