I suppose this would be a decent way to kill time during a break at work or something, but as has already been said, there's no sense of reward for creating a world. I messed around for a little bit, and upon having realized that the extent of this game is essentially blocky doodling, I quickly lost interest.
Some form of interactivity would add at least a star to this.
Also, why not have several layers for tiles, so you can cut down on the number of preset combos (ie, grass with stone, ladder with wood), AND increase the breadth of what the player can create? Even having fore and background layers would make it a lot more interesting to create stuff.
All that being said, the game does look fairly polished and neat, and I really like the minecraft-iness to the tileset.
The ability to see and rate the saves of others is a really neat concept, too, though this still remains lacking due to the aforementioned level of interactivity.
All in all, I wouldn't mind seeing this expanded upon, and believe this has a decent bit of potential.