I would argue that Gatsby is great because he epitomizes the promise of the American Dream. Basically a regular guy from a humble Midwestern background, Gatsby has come so far in his relatively short life. He's set himself a number of very ambitious goals in life, and by and large he's achieved them (although Daisy remains tantalizingly out of reach). One might certainly express disapproval at the way in which Gatsby became so phenomenally rich, but he's no hardened criminal. Despite his ill-advised involvement with bootlegging, he's still a fundamentally decent man: he's generous, companionable, and emotionally honest.
Gatsby's also great in that he's quite a character. Whatever else you might say about him, Gatsby's certainly someone you're not likely to forget in a hurry. That he's able to be such a memorable character without imposing himself on those around him—unlike, say, Tom Buchanan—makes him all the more remarkable.