I must be in the minority here. I think credits work very well. Possibly I don't experience the downside since at any given time, I generally have far more credits than I can use. Though I don't publicize it, I occasionally randomly give people a bunch of credits, especially if they are credit poor but want to do some AtAing around good questions. Earlier on, I used to do silly things with credits like attempting to blow it all in a few days just for the hell of it, but now that they are kinda worth something for promoting stuff, I treat them a little more seriously and avoid frivolous uses.

They are not a gamification mechanism. They are a quality control mechanism. So in that sense, they are a bad gamification mechanism. Just like apples are bad oranges.

Just because the mechanism involves an artificial currency does not mean it represents "gamification." Do dollars "gamify" the real economy? That's silly.

To qualify as gamification, the mechanism's primary design intent must be to increase engagement, amplify social proof and imitation effects (badges etc.) and (in the uglier cases) patterns of addiction that amount to predatory manipulation of attention. Quora does have a few such genuine gamification mechanisms, including:

1. The use of infinite scrolling
2. The inability to turn off certain categories of the "red flag" notifications.
3. Visible upvote counts
4. The "related questions" sidebar feature

All these are gamification mechanisms because they can suck you into endless clicking via a feedback loop that is too short (less than 1 second) for your brain's executive attention to interrupt the hijacking.

But credits aren't one of these mechanisms. There's nothing you can do at a sub-second scale involving credits that can suck you in. All credit-related UX elements take at least 3-4 seconds to do, and at least minutes to days to generate feedback. They are always under conscious control. You cannot unconsciously/automatically promote things using the slider for instance, because there is no instant feedback.

Before the credits scheme, there was far too much junk AtA activity. That's largely gone now, and you can also manipulate your AtA rate by moving your rate up and down. When I am busier or particularly annoyed by the Quora community, I move my rate to the max. When I am more bored, I lower it.

Question quality has also stayed constant, which is a significant achievement while growth is going on. I think this is largely due to the barrier to entry of credits. People do not appreciate this because it is a case of constant performance under changing conditions.

I haven't checked, but I suspect the ontology has actually gotten less noisy since the credit mechanism was added to it. Before, people used to add all sorts of junk tags to things. Having to pay 50 credits to tag something, and being forced to tag it to add a question, is actually an anti-gamification mechanism. It arrests an otherwise unconscious OCD activity.

Finally, the use of credits to promote questions and answers is also doing wonders for quality control. I often pay attention to promoted questions. I pay particular attention to answers promoted by somebody other than the answerer. My stream is much cleaner now.

Good job Quora team. I am frankly shocked I've stayed engaged so long, through busy/slack periods, over nearly 2 years. Though I have my complaints (especially the Boards feature..., and I am starting to come around slightly), it is impressive how much of Quora design is best-faith rather than cynical and predatory.

The above-par level of ethics in the design philosophy actually impresses me more than the design skill. While the design is certainly not pure as driven snow, ethically speaking, it is at least a couple of standard deviations above the norm for the Web.