Depth. Not just mining, but the entire game. Theres more to do than you first expect, places to go, people to z. As a free web game it's easily among the best, but with the thought of it being an unfinished game it kinda drops the will to continue on for too long in the game, as I know I'll be starting all over again. Even with the ability to transfer saves, the game might change before its full release. Stats might be rebalanced, story elements altered, new enemies to see that wont matter to a player further ahead in the game, etc.
Regardless, this was a very generous demo. Hope I don't ever have to mine this game up from unfinished hell.