Most of the great things he has tried to do have failed due to lack of bipartisan support. Social Security reform, for example.
But for my money, the greatest thing he's done has been to turn terrorism into the global clash of civilizations it should be. Nations can't pretend terrorism is a local problem with local law enforcement solutions anymore, as they did in the last century. In this century, they have to confront terrorists and state sponsors of terror directly, and make real policy decisions. He's also established a precedent of preemptive strikes and forced regime changes for state sponsors of terror and bad actors on the world stage. And he's dared to attempt a fundamental change in the paradigm of the Middle East. It's a bold and risky experiment, and it might fail, but I honestly believe it needed to be tried, and I honestly believe that he's a great man for trying it.
The second greatest thing he's done has been to run his entire administration with a complete disregard for the media portrayal of his work. Ironically, the most frustrating thing about his presidency has been his consistent refusal to make his case clearly, describe in detail the successes of his policies and his troops, and counter the spin of the opposition faction. He obviously prefers to do whatever he thinks is right, regardless of what people may think of him, rather than spend time trying to change people's minds about him. While it annoys the hell out of me that he rarely speaks up in his own defense, and never goes to great lengths to describe his successes, I think it's great to have a President who is so painfully media-ignorant and PR-unsavvy.