Addiction.

Feel free to build whatever you want.
Do it as long as you want.
At any time you want.
No goals to reach, no time limit.


A game which comes into my mind when writing this is Sim City. Part 4 of the series kept me entertained 24/7 for full 2 weeks!
No game has done this before, no game has done this afterwards.
When watching Minecraft videos I can feel why it has become such a success.
While it actually is not really like Sim City, it has some important parallels like building stuff as much as big as long as you want.