The UI is easy to use, but I wouldn't call it simple. There is a lot of information, and there are a lot of ways to interact with the content. Every comment has at least seven clickable links and three pieces of metadata, compare with HN (four and two.) They just did a good design job so it doesn't seem overwhelming.

The UI probably has something to do with the composition of the userbase. reddit feels not just interactive but malleable. That appeals to geeks raised on videogames.