I hope the graphics don't suck, like they do in Minecraft. I know it sound superficial, but graphics do matter. Minecraft succeeded despite its graphics, not because of them.
Firstly the game is written in Java and hogs enough CPU/RAM as it is. If they had started making high quality assets they would have had to maintain that standard throughout which would have made it much more difficult to iterate quickly which is something that minecraft is all about.
I think of it like Lego , Lego kits are nowhere near as pretty or detailed as other proper model kits but they allow many different possibilities.