I'd say the main thing to keep in mind is the topic. Put simply, the more niche/specialised the topic, the higher the quality of discussion will usually be.
And this is because those niche topics tend to draw in people who want to be there, and who want to discuss the topic at a reasonably civil level. So instead of being a megaphone for every crank with a Twitter or Facebook account to rant about flat Earth style conspiracy theories, they tend to be for folks who want to actually come to an agreement about things and discuss the intricacies of the topic.
It's why Hacker News topics on highly technical issues tend to have much better standards of discussion than those on political ones.
Of course, some topics are just gonna be hell regardless; any site or community dedicated to discussing politics, religion, conspiracy theories or other such hot topic subjects is going to devolve to Reddit like standards in a matter of days, if not hours. If your topic is something that's central to everyone's life and which large groups disagree on to the point of thinking their opponents are 'evil', then it's probably not going to be practical to hold a reasoned debate online.
You'll also want to think about incentives in the design of the system. Twitter for example is basically designed for quips and pile ons due to the character limit, quote tweets, various communities who hate each other being forced to interact, etc. Meanwhile something like Reddit can become an echo chamber due to the voting system, since anyone the majority disagrees with will be downvoted into oblivion and driven away.
So it may be worth looking at other setups here. Make voting more granular and based on aspects of the content (like whether it's informative, funny, creative, etc) rather than just 'is this good' or 'is this bad'.
Finally, moderation is hugely important here, and most sites get it horribly wrong. Have people with experience in the topic and decent people skills do the moderation rather than outsourced contractors, be consistent with how you enforce the rules to avoid favouritism and other clique related issues, and (potentially) be willing to actually hire your staff to do this full time if need be. Also be willing to come on and make changes here if things aren't working out, and you feel the team isn't doing a good job.
As for your questions at the end:
1. Yes it's possible for an online community to keep itself in that sweet spot, so long as it's fairly niche and isn't dedicated to sensitive or controversial subject areas.
2. Hacker News is somewhere in the middle, though getting closer to Reddit as time goes on. Dang does a really good job here when it comes to moderation, but the increase in politically charged topics is certainly bringing in some less than intelligent discussion, especially for more mainstream areas.