Theoretically, I think it can exist. However it would take a fair bit of luck for the core group that grows up around it being diverse in their opinions but also open minded.
Unfortunately most people these days are more interested in their initial opinion being right than in having real discourse that everyone can learn and grow from.
Political conversations that don't devolve into idiocy and hatred are the exception, and not the rule, in any forum or group I've ever encountered.
I think if your theory involves mass-exclusion/banning you could possibly end up with a place like this. But the conversation would actually lose an important voice at that point. The angry people are (often) angry for a reason, they're just usually pretty bad at making their point civilly.
Some of the most civil (and then again, some of the least civil) political discussions I've ever seen on the internet happen on places like Facebook, where people are friends, and where almost everyone uses his/her real name. I think those two factors are very important: 1) real identities, 2) some form of reciprocal relationship among all parties to the conversation.
If you look at a political debate from a game-theoretical standpoint, a debate among friends is a multi-stage game. You don't want to go nuclear right off the bat, because when the dust clears, you'll still want to be friends with your opponent. There's a continuity to the relationship. There will be a second, third, fourth,...,500th "round" to the "game."
I'm not sure if this situation is any more likely to yield productive discussions. But on average, it yields more civil, less abrasive discussions. On the downside, it can often result in echo-chamber conversations that never really get interesting.
Conversely, the worst and least civil debates I've seen have occurred on long-standing message boards or communities, wherein the members are anonymous, but they're known for their handles. These members have "known" each other for years in some cases, but they don't really know each other, and they have few qualms about unleashing the flames. Especially when they feel their reputations or credibility within the community are at stake. These users have created identities for themselves, and paradoxically, they'll often defend those proxy identities more ferociously than they'll defend their real identities.
Tl;dr: when people assume group or tribal identities, you get worse flame wars and less substance. When people assume individual identities, you get fewer flame wars, and possibly more substance.
I really want a place for a _structured_ political or philosophical discussion to take place. The same way that Stack Overflow placed a lot of structure and moderation around a Q & A panel.
Thats a g good question. I do have some good debates on Facebook but wish I could have it isolated from all those of my friends who aren't interested in politics.
I sometimes wonder if such a place can exist. Outside of friendships, I kind of don't think it can.