Finite amount of water. This gives me an idea which I doubt could be done. A full rain cycle. Evaporation, clouds develop, it rains, ground is saturated and lakes, ponds and rivers are fed, water moves through the land down to the sea (it would be great if there were small seas).. repeat. That way, if players take water from an area (a pond), it affects areas downhill or down the valley from them. If you drain a pond, you would have to wait for rains to fill it again. Also, draining water from areas would affect the fish etc.. living in that body of water. In the summer, droughts could cause some serious problems for players farming, giving reason to plan ahead and create water storage systems but you would still need to manage your water use throughout the year. You could sabotage your enemies in the long term simple by draining an uphill pond, thus more reason to think defensive, not just offensive (if the server had pvp...). I wish...
"simple mechanics that allow the player to create complex systems" exactly.
Yeah when I said torches scare off predators, I meant animal predators, no monsters which of course are more cunning etc.. and yeah, monsters would probably be attracted to fire for sure! Good stuff! Wish I could fast forward a year or so to see what the devs have done haha