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In my opinion, exploration in this context is extremely important. The guided tours are nice, but I'm confident that you don't want to limit people's use of the siteAlso, "You're doing great! To continue, you'll need to register or sign in. Otherwise all that awesome progress you've made will be lost. Sign In\Register (it's free)" was a complete surprise, and felt like a roadblock (in spite of the "it's free" message.
If a user types through enough of the pages to get this message, they probably are enjoying the site and want to keep working with it. Why not include a "Not now" option in this dialog?
That way they can choose to join when they step back when they are done and say "Wow, that's a nice site! I want to come back later and keep working on this. I guess I ought to register!", rather than a "Meh, I've only invested 2 minutes, guess not".