Earlier quoted context omitted.
I think that would-be users should be shown a comprehensive experience without having to sign in; if things don't work when they first go to the site, then they probably won't go back...
some features are specific to the users and hence they need to sign in .. would a demo video work instead?
For example, if you don't sign into facebook, you can't see a wall or profile or anything, but if you sign in, everything is available.