Both my Master's and Bachelor's degrees are in Philosophy. My undergrad thesis was on Nietzsche. One sugar, low-fat milk. Design, navigation, technical performance, content, layout, etc. If the UE isn't a good one, people are unlikely to return.
Studies show that visitors make judgments about web sites in the first few seconds.
It's a great challenge to provide a UE that convinces them to stay. Information architecture plays into UE because sites must be easily and intuitively navigable. Say you build a site with frequently changing content. A user reads a story that's teased on the home page. A week later that story is no longer featured on the home page.