In my experience, unusual game mechanics breed terrible edge cases. It's difficult to handle these gracefully and consistently. I'm developing a new Flash game called Toaster Bro: http://blog.zacharcher.com/2009/03/20/toaster-bro-alpha-10-d...
The basic concept is simple: you have a magic wire. You plug outlets together to produce various effects. Hopefully it's not evident to the casual user, but this has been extremely difficult to smooth out. There are many types of outlets (not just electricity); wires can stretch between rooms (but only the player's room receives tick events, so the toaster rooms trap you inside, preventing you from remotely plugging in the toaster); there's a full scripting engine but you still have to handle all the state changes, ...
To keep this game simple, I made an early compromise: only one disconnected wire is allowed at any time. If you start drawing another wire, the original vanishes. Otherwise it becomes a topological nightmare, as users want to connect the loose ends of 2 wires, it's just very confusing to play, or understand what's happening.
A friend asked me if I could put logic gates in the game. This is probably superfluous (couldn't you just run electricity to the final destination?) but this could be used to break the system, because you can plug electricity into the logic gate's output. What now, bitches?! So you need even more rules, then.
Also I think basic "warfare" games are easier to contemplate, because we're all familiar with throwing pellets at our grade school friends, and whatnot. Most of us didn't grow up warping the fabric of time and space, so it's hard to think in that context ;)