I just started using a cast iron that I got from a friend (I don't believe it has ever been cleaned/seasoned/any special treatment). So far, everything I've cooked in it (steak, fish) has come out really good, even though the typical traits of a good cast iron are definitely not there (rust, uneven surface, no shine, etc.) In what ways would food cooked in a well kept cast iron differ from a poorly kept one?
I would say that rust, an uneven surface, and no shine are the traits of cast iron that has not been cared for. It sounds like you are using a seasoned cast iron pan. You do need to maintain it, or it will not stay that way. (There are lots of questions and answers on the site about cast iron. Just use the search bar.) The benefit of cast iron is heat retention, and the even conduction of heat. A well-maintained pan will also develop a virtually non-stick surface. In addition to having to deal with rust, a poorly maintained pan would lead to more sticking.