Lego is very fun (well I think so), but an app to just play lego isn't going to do as well as a lego app with game mechanic and goals to accomplish.

Minecraft has some simple mechanics. You can collect stuff. You craft stuff. You have to survive. That makes it fun to get into. And yes the creativity and community builds on that to achieve some amazing things which amplifies the fun, and at that point you can ignore the survival/goal part of it. It's quite a bit different from a virtual world like Second Life.