Is the downvote-answer feature necessary on Quora?

Yes.  Without downvoting, I think you open Quora up to non-constructive answers which weaken the strength of the site. See What are good and bad reasons to downvote answers on Quora? That question has a number of answers that get at this point. That question and its answers also support the secondary point that Justin Bishop is making — which I agree with — that unfair/unreasonable downvoting is demoralizing.

When will Quora release a BlackBerry app?

It's really hard to say since we're so early on and haven't launched yet, but I do think that mobile interfaces are really important (and are growing more so rapidly) and make a lot of sense for this kind of tool, and behind the iPhone, Blackberry is probably the next most important mobile platform.

What does an "endorsement" do on Quora?

We reintroduced endorsements recently (Reintroducing Endorsements by Abhinav Sharma on Quora Product Updates) with slight changes to the original model. It remains a public signal of trust in a person about a topic. This is how an endorsement works now:

  1. You can endorse a person about a topic they know about, in which case they get a notification about it, and your endorsement is added to their list of endorsers on that topic.
  2. If you endorse a person for a topic they have not yet added as something they know about, then they get a notification saying that they have been endorsed and asking them if they would like to add it as a topic they know about. If they accept that they know about the topic, then your endorsement is shown in their Knows About section.

    If however, they say that they don't know about the topic, then the endorsement is not shown on their profile, and they are not notified or further endorsements about that topic.

We encourage endorsing people for demonstrated expertise on Quora or as a form of encouragement to write, on topics you know they're knowledgable about, even if they haven't actively contributed yet.

What are the main reasons for limiting the number of beta users for Quora?

My understanding is that the Quora staff wants to carefully mold and develop the dominant cultural norms of the userbase.  Technical scaling is much less of an issue than the social engineering part, where developing the right type of community and userbase is critical to the product being useful.

How do I invite people to Quora?

For both Gmail and Yahoo, Quora uses OAuth [1][2] to request authorization to access a user's email contacts.  If you're not familiar with OAuth, what this means is that Quora never actually handles the user's password.  Instead, the user is directed to either a Gmail- or Yahoo-hosted authorization webpage where the user logs in and grants access to a third-party application (in this case, Quora); the application then gets a short-lived, authorized request token with which to make API calls for the user's data.  Both Gmail and Yahoo expire their OAuth request tokens after an hour.  Using OAuth protects the user's password and also restricts third-party API access by Quora to only the users' contacts data.

Even for other email providers where the public OAuth standard is not supported, we value our users' privacy and security and do not save passwords.