Toss that sketch over the wall
One of the great features of Etherpad is that you could create a new document with one click on the homepage, and share it with others by mailing them a URL. No signup hassle, no up-sell, just click and go. I left this in, of course, when I built Sketchpad, but the real action now happens inside the Studio, which requires account sign-up/sign-in. I added some windows to the studio so outsiders can peer in, but there is still a real wall between the sketches created within the Studio and those created from the big button on the homepage. People do some great work outside of the studio, only to realize afterward that no one will see their creation. So I added a new feature that I think is a pretty nice solution: The ability to throw a sketch over the wall into the studio. To avoid metaphors and keep things clear, I just call this "save for later."
When you click the link, you create an account, and a copy of your sketch (with a pointer back to the original) is added to your new sketchbook in the studio. Now others can see the sketch, they can see who created it, you can give it a name, and make it a part of the studio. I think this is a subtle but effective way to move from the great one-click-to-start sketch to the real action inside of the studio. If you try it out, please let me know what you think.