I'm not sure "more moderation" is what caused that anyway. I wasn't a moderator before the forum changes, but from what I noticed then, I think the mods had to get involved a lot more often than we have to now. There was a period of dramatic change, but the entire community, minus a single handful of individuals no longer on this forum, has embraced the changes in the end, and I think the site is a far more positive place as a result.
When potential new members join a site, I think they're much more likely to stay on it if their first impression is of a forum calmly discussing Diplomacy or *politely* discussing the news, than if their first impression was the sort of rabid debate with extreme hyperbole and vitriol thrown about that it used to be.
I also don't think that the forum is now solely home to "centre-lefties regurgitating jokes"... I for one am no centre-lefty, though I rarely have time for prolonged debates here. There's still a varied array of views, but people can civilly discuss the views now, rather than having a ratio of 1 pt view: 5 pts ad hominem.