I checked a number of reasons.
I moderate at a large (non-political) site with ~650K members, and my first impression of this site was that it well moderated without being restrictive. When you moderate for so long, it is very difficult to visit a forum regularly that is poorly moderated because you spend all your time instinctively heading for the edit or delete button only to be disappointed that you are suddenly powerless.
There are plenty of sites out there that are either completely one-sided, or attract mostly the hit & run posters of topics, those that just respond with an insult (with no reason or discussion), or posters that respond with the same message in spam-like fashion regardless of the topic of the thread.
This site was the only one that I found where members actually discuss a topic and you can get into a debate deeper than just spouting the party talking points back and forth. There are still rabid "party line" participants, but they generally stay on topic and don't stifle the discussion.
And members here throw around much more intelligent insults.
[EDIT] Forgot to add - Palin got me pumped about politics again. Was always a political junkie but the fiscal actions of the GOP under GWB left me feeling that I was a man without a party. Even fiscally conservative friends would blindly support the GOP even when they acted like DEMs. The Palin selection gives me hope that McCain is going to govern using common sense more than the typical party politics that have made both parties more similar than different.