There's a fundamental difference: a slim group of Digg users push everything. On Reddit, the circlejerk is encouraged by the users. I've been a user for 3 years (just deleted my second account and banned the URL yesterday), and people like those posts. I upvoted a few myself before I felt it got out of hand.

A lot of people discover atheism and politics on Reddit, and it's wonderful for them because it's showing them new things. For everybody else it quickly becomes a problem. (It's a problem on HN too, to be fair: I've gotten into a few debates here with atheists who think religion is wholly awful, and their immediate reaction is often "if you don't hate all of religion, you're buying into a cult/a conspiracy".)