Trying to sum up... and suggestion
Much of the below has been said here and there but a few points to start:-
1: The root cause is NOT Captcha, it is the nature of humanity that is the problem. If we were not a species of "nest foulers", defecating on the very things that we try to construct for ourselves, then in most cases we would not need any such protection as Captcha tries to provide. Nobody wants Captcha to need to exist.
2: Captcha is not necessarily unfit, as a general concept, it is the implementation that has short-comings. Most of the alternatives suggested have been Captchas in concept, just with different tests.
3: Email verification is NOT an adequate solution by itself, for many reasons mentioned already. It is an excellent 2nd or 3rd line of defence during initial registration.
4: Whilst some sites can be overly paranoid with putting a visitor through repeated Captchas there are good reasons for Captchas after logon E.g. defending against visitors that unknowingly having bots (viruses) that use un-closed sessions when there has been no user activity for a while. Also defends against cheap human labour registering and/or logging in and then running an automated system.
5: There is no single solution that will suit all, as the various suggestions have shown. All of them will fail if a visitor is not 80%+ able at least one of sight, sound, mental abilities, memory, dexterity etc.
On a sites I maintain, using off-the-free-shelf plug-ins, they usually ask random Captcha test type from a range of installed ones including some along the lines already mentioned and usually requiring a small degree of human logic (depending on the nature of the expected site visitor). That is not perfect but does address some of the perhaps less thought through objections.
Taking that Captcha, or an equivalent gate-keeper needs to exist. And that the gate-keeper needs to cope with the abilities and limitations of any visitor...
- When a user registers they select the types of Captcha tests they wish to avoid e.g. (_) Visual, (_) Colour, (_) Audio, (_) Literary, (_) Mathematical, (_) Intellectual
- Captcha mechanism to operate from a pool tests that are classified by the above styles so that excluded types are not displayed for that user. If you are impaired in sight or vision then you will be given tests that avoid your specific limitation(s).
- As new or updated tests are released in to that pool the site will make uses of them i.e. the site, and visitor's profile, are NOT configured to use any specific Captcha test.
- In addition general quality of Captcha tests need improving, use of standards, review process etc. for new added tests.
This would need to be a free community system available as plug-ins for all major website systems such as Joomla, Drupal etc.