Yet it is still considered part of the series due to the appearance of Clint Eastwood, and the similarity of story (all three concern the acquisition of large amounts of gold by The Man With No Name playing two sides against each other).
Secondly, the budget is considerably larger, and therefore, for the first time, there are huge sets with many people in them.
This movie is the most famous of all of Leone's work, largely because of the incredible Ennio Morricone score featuring the modulated screaming in the main theme that is the most instantly recognizable western theme of all time. There are only a few pieces of film scores that have risen beyond the films they are associated with to become part of our culture (the stabbing music in Psycho, for example), and this is one of a very select group. Morricone's theme was the attempt to recreate a hyena's cry, and while most people don't recognize a hyena, the pure energy and aggressiveness of the scream is a powerful motif in the film that Leone uses to good advantage, punctuating scenes with them throughout his film.