[luckylion]

Have you tested that hypothesis? In my experience, the real annoyance comes from people that are very persistent and aren't easily deterred by requiring a small effort. Captchas slow them down (by the time they need to solve the captcha), but they'll also slow down everyone else.

Adding effort so you have to really want to comment may be a good idea to reduce the amount of comment, but it might also deprive you of valuable comments because people don't care that much about sharing some information that they'll jump through hoops to do it.

[comntr]

That's what I'm trying to test. Last time I've got 800 mostly meaningless comments.

The idea is to stop numerous zergs: you can't ban them one by one, but a little bump on their way can do the job.

To avoid annoying more serious commenters, the captcha should be as simple as it can be. So asking to add 321+625 would be too much, while a question like 2+4 doesn't need any thinking, so I chose two-digit numbers.

This is a small social experiment. I don't know how it'll go.