I am not even sure most of the people would enjoy non-censored bitcointalk forum, and they wouldn't like to live in world without any censorship, it's just a pipe dream.
I think it doesn't scale well. Spammers will be unlimited, and even if there's a system in place to vote out spammers, new users will never get through if spammers create millions of new accounts per day. If you accept users on invite-only basis, new users have the same problem.
And I'm not even going to talk about hosting... because in the end there always has to be someone behind the scenes managing the server.
A truely decentralized forum won't have any centralized hosting. There could be nodes, but each user would keep their own copy, and broadcast it to other users like bittorrent.
- There has never been a truly decentralized protocol where each user can have equal voting power.
Bitcoin tried, by basing voting power on CPU power. That didn't last very long. And even if there would be a magical protocol that gives one human one vote, votes would simply get sold.
I'm afraid you don't understand the fact that a forum has WAY MORE traffic compared to a decentralized payment system exchanging a few numbers here and there.
So no, it's not going to be more lightweight, quite the contrary!
After the IFD (Initial Forum Download), all each user needs to do is download all new posts. That's currently only 4 posts per minute. And each user (on average) will have to upload each post to another user (in a decentralized way, like bittorrent). That's actually the beauty of a decentralized system: bandwidth scales very well. If one person shares 1 GB on a server, each download consumes 1 GB bandwidth. But if he shares the same file through bittorrent, he doesn't have to spend much bandwidth anymore because every downloader starts uploading too.