Very fun, even though there are no goals. You have just wasted two nights of my time. ;D
Here is my first construction, a gate and a boat: http://www.youtube.com/watch?v=aCiRR5opnGo
The gate appears to be as high as possible, because the game did not allow me to put any more blocks on top of it. I made the boat by first filling it to drive the water out, and then etching the inside.
Cooperative multiplayer mode would be nice. Maybe so that first in a map editor somebody creates a plan that what to build, and then everybody will see in-game the blueprints (as a wire-frame model) of what they are expected to build.
In a competitive multiplayer mode, it might make sense to restrict the use of some materials. For example, rocks you would need to first collect from a mine, before you could place rocks somewhere else. Wood you could use unlimitedly, but the enemy could burn it easily, so for any strong buildings you would need rocks.