Really enjoyed this Smashing magazine article on E-Commerce user interface design. Of course it is not omnipitient and does not go into all of the reason why users abandon the checkout process.
I know from my own experience I have abandoned a checkout because it was not HTTPS. Some websites only become HTTPS very late on in the process. As soon as you start sending shopping information you should be on HTTPS pages to emphasise security. Security experts will be happier and non-experts who have been told about the Green URL bar will also feel a bit better.
Additionally I have been unsure about making a purchase I have proceeded through the checkout process only to decide that I dont really want the item. Now it could be argued that the persuasiveness of the website was insufficient at the checkout stage, but I feel it was not an easy of use issue and it would be difficult for the UI to have lead to conversion at that stage.
Of course 60%+ does seem like a high loss level and certainly leads me to think that some of it is difficulty in completing the purchase, some loss will always occur.