I think what makes Smallville work is its ability to connect with fans and casual viewers. Most episodes, particulary in the early seasons could be watched without having seen every episode up until then. I think because of Superman's impact on popular culture, people can easily work it out. Not just these things, but theres a big element of real life issues featured, including a lot of stuff that Teenage audiences can relate to.
Whats more, Smallville has had a definite end, from the beginning. Everybody knows how Smallville is going to conclude and where the story is going and the focus of the story is how Clark gets to where he is going. Throw in a load of popular DC characters and references and you've got a long running, successful show.