the smart way to do it is to allow you to make a theme, but to save it/share it, you need to sign up.

If the theme is just a throwaway, people won't sign up, but if the theme is really good and the person wants to save it, it makes the signing up easier to swallow (since you get value from saving it).

Otherwise, you end up with crappy noise in the sign up because people would just make a fake account to try it out.

Me too: I am unpleased by forced sign-up before I've had chance to try the thing I'm signing up for, so I didn't get beyond browsing demos. Some of them did look nice, but nah.