Interesting. So the principle of trust the players to sort things out tends not to work, then? Because I was considering leaving it totally free sandbox - no plot hooks or anything, but occasionally warp in challenges, treasures, enemies, or allies.

Maybe have a few rules along the lines of no [url=http://en.wikipedia.org/wiki/Grey_goo]Gray Goo[/url] [url=http://en.wikipedia.org/wiki/Flood_(Halo)]ty[/url][url=http://en.wikipedia.org/wiki/Replicator_(Stargate)]pes[/url]. Hmm. Well, the more about exploration and construction, rather than destruction and "winnning" the game, the better, in my opinion.