Two years ago we (UserVoice) removed the need for end-users to sign up or register. When submitting an idea, voting or commenting, they simply identify themselves with their email address – much like commenting on most blogs. This eliminated one of the biggest barriers for user participation: registration.
Later, if they want, users can confirm their address and add a password. This is optional, unless the user is an account administrator or is trying to access private content (in which case we do require to confirm and protect their identity for security reasons).
We also allow people to sign in with their Facebook or Google identity. Since those services return a verified email address, we can reconcile it with an existing user record if one exists. This eliminates another barrier for returning user participation: “Which 3rd party service did I sign up with last time?” and “If I choose the wrong service, will I accidentally create a separate user profile?"
Even if the user forgets, they can simply enter their email address and, if it’s protected with a password or FB/Google authentication, we prompt them with the correct method to use (while unprotected users don't need to authenticate at all).
Lastly, we removed the need to sign-in (or sign-up) before participating on the site. By asking users to identify themselves at the moment they want to participate and not requiring a registration process, participation is just as easy as leaving a comment on a blog. This, along with not requiring registration, eliminates the issue where new and returning users were interrupted by a sign-in/registration process in the middle of their existing workflow.
One thing worth noting is that this isn’t a one-size-fits-all solution for all online services. It works great for UserVoice and our customers since most users aren’t performing private or sensitive tasks.
I’m very excited to see more services focussing on creating great experiences when identifying new and returning users, and questioning whether common pre-conceived notions around these requirements are still necessary.