How do you reduce the barrier to becoming an authenticated user in Drupal? There are a few goto solutions: Email Regiatration, Login Toboggan, captcha on the user registration form, etc. These are effective and well tested solutions that allow new users to register and reduce the threat of bot registrations. Single signons with Facebook or Twitter are also viable options and arguably reduce the barrier with the lowest risk. Unfortunately they require third party account which doesn't meet the need for a user who wants an account only on your site.
In my mind Login Toboggan offers an interesting solution to this issue by creating a specific role for a user that can be created and logged in but not given full rights until they confirm their registration. Where it doesn't lower the barrier is that it still requires creating a username and password during account creation.
The thought experiment here is how much of the registration form can we strip away to get a user created without out overly risking bot registrations. Since email addresses are for the most part unique, is it possible to use only an email address to create an account without a username or password? The following is a flow chart that this kind of registration might use with a nod to Email Registration module. Proto code forthcoming.