With most eCommerce sites experiencing 60% abandonment at the shopping cart (according to MarketingSherpa research), it seems pretty logical to suggest that this is an ideal place to start when making changes to your site. In golf, many instructors advise their students to "start at the hole, and work backward to the drive" when improving their game. Yet the focus still seems to be on "front end stuff," like driving traffic (SEO/SEM), for which the levers seem easier to identify (add money and stir).
What site designers need to consider is "why," someone comes to the site, follows the path all the way to the cart and then bails out. It seems like such a waste of time, not only for the customer, but the online merchant, as well. When customers are asked why they leave at this point . . . many of their answers boil down to "unpleasant surprises." The shopping cart is the wrong time to find out, for the first time, about shipping charges or other hidden fees. It's also the wrong time to present endless forms for completion, which may include registration or signing up/opting out of special offers and emails. We seem to forget that when they get here, they are ready to go . . . and that if you delay them that they will really go . . . as in go away.
If you only have a little time to work on your site, look at the cart. It may be at the end of the line, but if loaded with barriers for visitors, it should be at the beginning of your plan for site improvement.