(WARNING: Minor spoilers below)
Sully didn't save the day in the end. Eywa did. The Na'vi were beaten, defeated, even under their supposed white savior's leadership they had been repelled and were about to suffer an irrecoverable loss that would have effectively been the end of any chance of them resisting the invading humans.
Then Eywa stepped in, in the form of hordes of creatures overwhelming the humans with sheer numbers (immense size and strength and in one case impenetrable armor certainly didn't hurt). The humans were overrun and all but defeated at this point, and only then did our white hero get the upper hand and bring down the shuttle-cum-bomber and the uber-gunship/mobile command center.
The only thing Sully brought to the table was outside-the-box thinking, only made possible by the fact that he was never inside the Na'vi's box in the first place. "Eywa doesn't take sides," says one Na'vi after overhearing Sully praying to Eywa for help in the climactic battle; the Na'vi would never have even thought to ask her, yet in the end it was only her help that made their victory possible! Sully also knew things about the human tech the Na'vi could have known - such as where to place a tiny hand grenade to bring down the big ships that are otherwise invincible to anything the Na'vi can bring to bear.
That said, however, Sully has an irrefutable advantage over both the Na'vi and his own people: Two distinct perspectives on the world. He grew up human, but then was assimilated into the Na'vi culture. While never truly becoming "completely" Na'vi (he'll never 100% integrate into their culture), he nonetheless gains an understanding of them and shares in their appreciation for the world around him. He in effect is able to think like both a human and a Na'vi, and that leads him to take actions the Na'vi would never have considered - such as riding Last [private], gathering all the tribes for a counter-assault, and asking Eywa for her intervention.
I've got to run, so this isn't as polished as I would have liked it to be. I apologize, and will clarify points later as needed.
Sully didn't save the day in the end. Eywa did. The Na'vi were beaten, defeated, even under their supposed white savior's leadership they had been repelled and were about to suffer an irrecoverable loss that would have effectively been the end of any chance of them resisting the invading humans.
Then Eywa stepped in, in the form of hordes of creatures overwhelming the humans with sheer numbers (immense size and strength and in one case impenetrable armor certainly didn't hurt). The humans were overrun and all but defeated at this point, and only then did our white hero get the upper hand and bring down the shuttle-cum-bomber and the uber-gunship/mobile command center.
The only thing Sully brought to the table was outside-the-box thinking, only made possible by the fact that he was never inside the Na'vi's box in the first place. "Eywa doesn't take sides," says one Na'vi after overhearing Sully praying to Eywa for help in the climactic battle; the Na'vi would never have even thought to ask her, yet in the end it was only her help that made their victory possible! Sully also knew things about the human tech the Na'vi could have known - such as where to place a tiny hand grenade to bring down the big ships that are otherwise invincible to anything the Na'vi can bring to bear.
That said, however, Sully has an irrefutable advantage over both the Na'vi and his own people: Two distinct perspectives on the world. He grew up human, but then was assimilated into the Na'vi culture. While never truly becoming "completely" Na'vi (he'll never 100% integrate into their culture), he nonetheless gains an understanding of them and shares in their appreciation for the world around him. He in effect is able to think like both a human and a Na'vi, and that leads him to take actions the Na'vi would never have considered - such as riding Last [private], gathering all the tribes for a counter-assault, and asking Eywa for her intervention.
I've got to run, so this isn't as polished as I would have liked it to be. I apologize, and will clarify points later as needed.