I'm going to catch a lot of frak for this from the Let's Play Cave Dwarf Fortress! crowd but some of the ideas this allows are just too funny.
Electricity.
No I'm not talking about lightning generators or robots. That's WAY out of line for DF. My thoughts is it acts just like a power-type, generated from special windmills/waterwheels/possibly some form of coal-burning building.
The diffrences are the following:
1)It loses no power in transit.(no sapping from gear assemblies/what have you. Though if you hook it up to a gear assembly then to an axle you would lose power, though I can think of no reason to do either)
2)It conducts along metal& water. Metal constructions, floors, walls, buildings, anything made of metal will get an amount of power in it and be considered "Live". Water does the same.
3)Live constructions are hazardous to health, low power levels merely stunning, very high ones capable of killing a dwarf on the spot( or goblins or whathaveyou)
4)You can't SEE electricity. Neither can dwarves, or anything else. "Live" floors, and "live" shallow bodies of water path as being safe, legitimate pathways to go through. For flying creatures? It genuinely is. Everything else smells deep fried after a few minutes.
Electricity, is basically one of the key forces in the universe, dwarves being stupid, should reflect this in their usage of it. I.E. It should be at least as dangerous to them, as to their attackers, if not more so, and while useful, the knowledgeable user is going to have to be very ,very careful to not make his dwarves all kill themselves with it.
In short moderately useful, particularly in building lever activated traps(use little 1x1 retracting bridges as ON/OFF switches) or moving power a great distance, but with some very significant draw backs.
NO Robots, Tesla coils or anything like that. Just a new type of power.