Well, for starters the cast of the movie is incredible. Scorsese has a knack for bringing out the best in the actors he chooses for his roles, and The Departed is no exception. Many people agree that this is the finest work by DiCaprio as well as Wahlberg, and I know a lot of people who held Matt Damon in much higher regard after seeing the movie, including myself. Alec Baldwin, Martin Sheen and even Jack Nicholson, all long time renowned veteran in the movie industry, turned in some of the best work of their careers.
Secondly, the pacing is great for a film that lasts over 2 hours. This has always been a trademark of Scorsese's. Goodfellas and Casino, both of which are over 2 hours long (Casino is a minute shy of being a full 3 hours), never seem to be as long as they really are, and The Departed is the same. Scorsese makes every moment in the film lively and exciting, and there's never a dull moment. Few directors pace their film btter than Scorsese's.
Next, while the plot may not seem revolutionary at first, it's still one of the best written in recent years. The parallel between Costigan and Sullivan is incredibly conceived, and the character development (always a a strong point with Scrosese) unfolds seemlessly.
Add to that it has the usual excellent editing from Thelma Schoonmaker as well as a fantastic soundtrack (another signature element to a lot of Scorsese's best movies namely Goodfellas and Casino), and The Departed truly is worthy of all its acclaim.
Revolutionary? Well, not quite. But it's definitely a masterpiece, no question there.