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.