CAPTCHA is bad for accessibility
It shouldn't really come as a surprise to anyone that by using CAPTCHA (Completely Automated Public Turing test to Tell Computers and Humans Apart) to limit comment spam on a blog, you are making it difficult or impossible for several groups of disabled people to post comments. The affected groups include people that are blind, have low vision, and those that have a learning disability such as dyslexia. The W3C explains the problems with CAPTCHA and examines some potential solutions in the Working Group Note Inaccessibility of CAPTCHA.
For anyone who is not familiar with the term CAPTCHA, it refers to using a bitmap image to verify that the user is human and not a computer program. CAPTCHAs are commonly encountered in comment forms on weblogs and in forms used to register for online services, and usually consist of an image containing distorted text which needs to be interpreted and entered into an input field in the form. Posting a comment or completing registration requires first verifying the text in the image.
This obviously makes it impossible for a person who can't see or understand the image to post comments. Including the text in the image's alt attribute is not an option since that would completely defeat the purpose of the CAPTCHA, which for blogs is preventing robots from posting comment spam. The robots could easily be modified to find and use the alternative text.
Besides having accessibility issues, the use of CAPTCHA in my opinion suffers from usability problems and should be avoided. There are other ways of preventing comment spam that do not affect accessibility.