Firstly, Alexander got lucky in many ways. His father built the outstanding army that Alexander then used to conquer. Had Phillip the II not died early then it is conceivable that he could have been known as 'The Great' rather than Alexander. Furthermore, in Darius he had the perfect enemy i.e. someone who was massively incompetent.
Secondly, Alexander's legacy has almost certainly been improved by his early death. By dying early Alexander wasn't able to make any many mistakes. If he had lived he undoubtedly would have done. It is often harder to hold an empire than to win it and holding Alexander's empire would have been hard. Remember Alexander was no strander to almost causing disaster. By forcing his men to keep marching into India Alexander had already come perilously close to mutiny and execution.
Thirdly, whilst Alexander seems to have recognised how difficult it would be to hold his empire but his solution seems to have been forced migration of cities in order to intermingle his people. These tactics have been described at proto-hitlerian by some historians. Had Alexander lived to implement them then it is likely that modern opinion of him would be significantly coloured.
