Idea: Trial/Demo/test-drive before sign-up to DuoLingo
One of the troubles with Duolingo is that you cannot try out the site before signing up. The instructional video shows more/less how it works, but nothing is better than test-driving it before signing up.
Personally, I dislike signing up to a new site if I can't see its usefulness before-hand. As an example I knew about duolingo one(1) year before actually using it, and the first time I planned to use it I changed my mind simply because I had to sign up/ create a new account to yet one more service.
Summary: A short trial/ summary before using the product(Duolingo) may attract users that would otherwise not take the plunge.
Thinking about how Duolingo works, it may be quite challenging to set up a trial version of Duolingo. It's my understanding that Duolingo is providing you a customized experience essentially from your first finished lesson.
Rather, a more detailed demonstration of Duolingo might help especially if it includes an explanation of why sign-up is necessary. Possibly a video demonstration of a lesson and an immersion translation.
P.S. I understand your hesitation with signing-up. I'd been told that a particular web site was good for learning Spanish. obviously not Duolingo. And, I had to sign-up before I actually could see the site, so I signed up and found out that it really did not meet my needs. But, now I am getting daily spam from the web site. Very irritating.