I came over to Open Cart from Zen Cart for one specific feature - the guest checkout. I think that it something that every site should have.
The whole problem (IMHO) is that shopping carts have historically evolved from the wants of web developers, rather than the needs of shop owners. Wherever did we get to the point of subjecting customers to having to fill out forms just to get a shipping cost, or instantly expecting them to have a pen and paper at hand to write down (and think of) a username and password.
Going back to the original point as to why folks should use OpenCart, apart from the much needed guest checkout, the admin area is much cleaner and easier to use than many other cart offerings out there. The code is also easier to work with and more logical than some of the obscure structures I have encountered.
Things I don't like about OpenCart is that there is only one template available, the 'others' I've seen are often just tweaked colours. You just get the one template supplied with OpenCart, take it or leave it...
Things like a PayPal Express checkout and shipping estimators should be part of the core program, not as paid commercial offerings from third parties. Once you start adding the cost of those paid offerings up, you can end up with a program that is more expensive than the commercial solutions that offer 24/7 support.
OpenCart is still undergoing its early evolution and I don't know whether its future lies as an open source alternative to the likes of CubeCart and Zen, or whether it will eventually position itself as a semi commercial offering?? As it stands at the moment, it is certainly a decent open source solution and Daniel and others have put a lot of hard work in to creating an easier option to the likes of the bloated Zen etc.
Despite the above gripes, OpenCart is definitely in the top five cart solutions for me.