I enjoyed Departed, but its not really a great movie. Its just a normal good and well produced movie (it felt pretty forced in parts, and not really all that similar to Infernal Affairs), nothing more nothing less IMO. Not a masterpiece, which what 10/10 would entail.
It was just typical to give it the Oscars (even if the other noms werent anything real special either), it was obviously more for his body of work then this movie, why the do it this way who the heck knows though. They also did it with Denzel, Training day wasnt his best performance, but they gave it to him then more an acknowledgement of his overall ability then that performance alone.