Besides genetics, and steroids, it's fairly easy to see why Arnold was/is as great as he is. He has incredible drive, determination and an iron will. Where others would make excuses, Arnold would find ways to get the job done. He will do anything, and everything to get things taken care. He never just pisses and moans about crap, he does something about it. He applies this to all in his life, business, bodybuilding, movies, etc, etc, etc., etc.
He has accomplished so much in life because of his great outlook. He came here with nothing, couldn't even speak english, and now he is one of the highest paid stars in Hollywood, a former champion bodybuilder (who is still the ideal for almost all, some 25 years after retiring.), business man, and potential politician.
His drive is what makes him great. He actually applies himself. It's the same with all people who accomplish great things. They stop at nothing, and never give up.