I'm curious to know how requiring certain pieces of personal information affect a user's likelihood to register on a website and opt-in to receive communications. Is there some "amount," of personal info where it becomes a significant deterrent for most people? At what point will people begin to abandon the registration process due to making personal information required? Any studies/articles are also appreciated.
Ask as much as you need to and as little as you can afford to.
The more information a user has to give up, the more work it required of them to register and thus the completion rate goes down. Asking irrelevant things will also increase the feeling that your application is trying to accumulate data on the user rather than asking things that will make their experience better.
The above has served me, personally, as a good guideline. Never ask anything that you do not absolutely need. If age is irrelevant, don't require it. If your website mails users items, you will need an address. If your users don't need a nickname, don't ask for one. If you need the shoe size only if user that buy shoes, ask only those that buy shoes and not everyone. Ask as little as you can - you can always enhance the user profiles later or offer voluntary fields user can fill in their profiles.
Not sure how relevant this is to your question depending on the site, but it may help.
However, from my experience, less is always more. Especially when it comes to registration.
It's not a binary issue. We know that the more information you request, or the longer your forms are, the higher your fall off rate will be.
My recommendation is to ask for as little information as possible in the sign up stage, and then try to encourage people to fill in additional information in their profiles later on.