I currently use Quora. It's closed nature has some disadvantages, though writers clearly can choose to publish openly. Many of those can be mitigated.

Everything is logged. Changes, edits, whatever are all presented to all users, who can see what happened and who did it over time. I think that's a nice feature.

One thing Quora has going for it is the Be Nice, Be Respectful Policy.

In terms of technical knowledge, the site varies. In terms of advice, or something one might call wisdom, it ranges from good to really great. There are some garbage answers mixed in there, but it's not hard to see and have a dialog about the good stuff.

Notably, Quora is a place where people can share themselves and their experiences unabashedly. Women have had some trouble there, as have some other regular targets of discrimination, stalking, etc... Quora has dealt with those quickly and is very, very interested in people not having any worries about discussions related to minority, gay, women, trans, and other sensitive topics.

Some of those discussions, advice, information are very high quality, frank, real, and I've personally contributed to some of that dialog in ways that did some material good for people really struggling, or fearful, unaware of their options, or possible outcomes.

While not perfect, the actions so far have produced some notable results. I am eager to see how Quora plays out, because having that kind of dialog and the tools given to users to manage conflicts and provide themselves and others "outs" to what would otherwise be messy, troll ridden discussion is something many people would value, and it's something missing from an awful lot of discussion online.

Some of us do share technical information. A lot of us share insight and experience or perspective that can be coupled with technical information. This has value too, and it's a little different from the more structured and highly technical Stack Exchange, and from the more noisy, often difficult to pick through general message board or forum dialog. This niche is also something I'm watching with interest.

There are some nice things too. One is non-English speakers coming to add questions or participate in answers or comments. People can help by making edits, or in some cases, by translating for them. I've done a few of those. Quora has a few students learning English on the site, and they ask often intriguing, challenging and notable questions about the language and use. Over time, that could be a great resource.

I think "sharing knowledge" is somewhat misunderstood. In a general sense, who we are, what our experiences have been, etc... are knowledge as much as math, facts, technical data is. The overall norms for discussion at Quora allow for very frank, real dialog. This can be worth a lot.

Go and search on how to give an awesome BJ, for example. A gay man answered that with such clarity and perfection, it's kind of amazing really. Or search on women in tech. Ladies there are posting up a lot of real experiences and advice that might be hard to come by and interact around elsewhere.

I could give a lot of other examples.

So far, I like Quora. I give it some time, and so far, I've gotten some good value in return. Some time is going to be required to see the more subtle aspects of Quora play out and then we might better understand where the value is.