Epistemic status: out of my depth
- The license should be opt-out (in fact I don't think you can legally force a license on the content created by authors without their explicit consent?)
- CC-BY would be a much better default choice. Commercial use is an important aspect of truly open source content.
- Even better to offer multiple license options on posts, so people can tailor it to their needs. I'm a big fan of how this is handled for example in arXiv or GitHub, with multiple options.
I notice I had a hair-raising chill when reading this part:
we are planning to make Forum content published under a Creative Commons Attribution-NonCommercial license
This made me feel as if you were implying to be owners of the content in the Forum, which you are not - the respective authors are.
I believe that what you were trying to convey is:
We plan to add an opt-out option for authors to release future content under a XX license
There is also the question of how to handle past content.
The simplest option would be to leave everything with their default option (which for posts without an explicit license would be all-rights-reserved under current copyright law), but add the possibility for authors to change the license manually.
A more cumbersome option, but that might help with increasing the availability of content, is some sort of pop-up asking for explicit permission to change all past content of current users to CC-BY, though I imagine that can be more work to implement and not clearly worth it.

Definitely appreciate the clarity provided here; I'm a huge fan of the Creative Commons licenses.
I'd put in my vote for dropping the Commercial clause; very biased, of course, but at Manifold we've really enjoyed pulling EA Forum content (such as the Criticism and Red Teaming Contest: https://manifold.markets/CARTBot) and setting up tournaments for them. We didn't charge anyone to participate (and we're actually paying out a bit for tournament prizes), but all the same Manifold is a commercial venture and we're benefiting from the content -- a noncommercial license might make us more reluctant to try cool things like this.
Also, for what it's worth, NonCommercial content cannot be copied to Wikipedia or any other Wikimedia project.
I don't understand. I thought Wikipedia was non-commercial!
Yes, but Wikipedia content is published under a license that allows commercial use.
I agree that a CC-BY licence is better (without the "noncommercial" restriction). I am chipping in because I'm not biased the same way as you are, except that I read content on the Forum much more often than I write it :)
The purpose of copyright was to encourage people to make creative works. We don't rely on financial interest to encourage people to post here—it's a prestige-based system instead, which seems to be working very well. Moreover, if we want the Forum to be a vehicle for disseminating ideas, copyright rules militate against that. I see many more reasons for switching to a CC licence than not doing so.
As for why I prefer not having the "noncommercial-only" restriction, it's because I don't want people not to reuse work from the Forum (with attribution) because they're unsure whether want to do with it counts as commercial. We should err on the side of sharing info freely.
Update: I do, however, agree with those who think that the CC licence should apply only to new threads. It would not respect the consent of previous contributors to put a CC licence on content already on the Forum, and I'm not sure it's even legal to do so.
Thanks! Good point that we are pretty mission-aligned with some commercial ventures and want to be cooperative with them.