I've been prototyping some stuff for the ambient cellular automata game, and it never ceases to amaze me how much like alchemy messing with cellular automata is. Too much
CREATE_THRESHOLD and everything explodes in brilliant white. Too much
DESTROY_THRESHOLD and it all disappears.
It reminds me a lot of the precarious nature of our own universe, where the various phyiscal constants would prevent us from existing at all if they were only a little bit different. After messing around for a while, I got the above parameters to make something fairly lifelike and aesthetically appealing. It certainly wasn't easy, though!
From a game design perspective, I think a big challenge is going to be minimising the amount of arbitrary fiddling while still allowing for a lot of flexibility in changing the automata behaviour and eventual results. I can now fairly reliably explain why gods are so capricious in all our mythology: they're irritated that they spent so long tuning physical constants and the universe still doesn't work like they want.
Also, if you're the cellular automata I just made and you're sentient and coming to this page for answers, I'm really sorry.