The site looks great. However, I agree with apsurd. If you first let users play around, it shouldn't take long until they're hooked and much more eager to sign up.
I may be the only person with this question, but if I use Twitter to sign up, will it have any impact on my timeline (like an automatic "I just signed up for Infogr.am" tweet)?
You don't need to save work for non-logged in users. I just personally think it's very powerful to have a way to demo the app.
Alternatively can you set up a dummy demo account that new users can use?