This is an interesting question that I'd love to hear other opinions on.

My idea that I haven't gotten to implement yet: Make it possible for the user to get started without logging in, then force them to create an account after they've started playing around.

The particular application that I specifically want to try this on is going to be a document-oriented application. I'll let the user jump in and start creating something without creating an account, but then make the user create an account (or log into an existing account) to be able to save their work. If you've already spent even two minutes creating something, having to type in an email address and password seems trivial compared to having to start over again if they want to come back later. (That's the theory, at least.)

The key in my mind is to get the customer invested in the application (even slightly) before forcing them to make an account.

I feel exactly the same- let users use and require sing-up for saving data etc.

But for young applications, it is important to get in touch with the users, and try to engage them-

There are some start-ups dedicated only to automatically send users email after some conditions, and most of the service I use do it manually too (like sending you "we noticed that after you built your form you didn't publish it for a month, care to tell us why?")

If you don't know who your users are, how can you get the negative feedback?

Getting feedback only from the people who sign up is going to be skewed to the positive side, and you'll have a problem realying on analythics for guest users (because some "guest users" are can be the same user- from different machine, or after cookie clean)

Depends on the product I think, however I do enjoy apps that I can "tour" without signing up. Especially if the sign-up isn't quick.

If you can build value in your product without forcing me to login, I'm more likely to want your product more because I've been sold on it. I think thats important, I should want to sign-up. This could be via a product tour, guest account, even a demo video. Ultimately I'm window shopping so give me a reason to come inside.

No matter what though whether you allow anonymous users or require a sign-up, measure the funnel between guest-user and registration. Find out where your users are bouncing and where you could improve the onboarding. Is the homepage persuasive enough? Is the sign-up page itself losing users? Measure, iterate, and repeat. I'm sure there's quite a bit to learn.

We do not require sign-up to use our form builder web app. You can access all of the pages without login. You can create a form, copy it to your web site and get submissions by email. (The email address can be entered on the form setup.)

This gives people a chance to try the product without making a time investment. We are getting close to 1 million users (not including guest users) so it seems to be working pretty well for us.

Our product is JotForm if you would like to see how it works. www.jotform.com

Awesome, seems like it is working well for you.

How did you get feedback/nisights about users that started to use your product, but fall of the funnel before entering the email address from some reason?

You should provide a good reason for visitor to become a user. never block everything for guests(like many websites out there), you should provide a great demo or any strong reason for people to become a user. That's clear nobody will fill forms and do activation process for just a sentence that you wrote on your home page. "Find Friends" - Signup "Live Chat" - Signup

Not requiring signup makes development massively harder but I think it's worth it.

All of the websites competing in my space require a signup except mine. Their products may be way better but I don't bother to check and neither do my users.

In fact I think you can make a viable business just by taking popular websites and working out how to implement them without user accounts.

Can you consider saving users data via cookies or ip? That way the next time they come back their stuff is still there. Now on the second run and beyond, you can remind them that they can take their data with them to other computers, or privately, if they create a login.

Some people might have dynamic ip addresses. Cookies can expire and get purged very easily. Same with local storage.

Also, people use many machines (desktop, laptop, tablets)

I don't mind signing up beforehand as long as the service comes with the option to delete my account and data at will, assuming it doesn't delete itself if I decide not to sign up.

And I don't mean "disable your account until you come to your senses" but "it's like you were never there."