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.

Thats true:
the idea of an open ended sandbox game,
without a specific goal.

There are next to the SIM games, also trading games like
Patrizer 2, Die Gilde, the X-Series, who let you freeplay (almost without a limit)

Very good post damocles. I had not thought about some of these points at all. I think I may have to read through this again (saved it) before starting my next game...