I think what people are getting at when the mention powered minecarts is something similar to what rollercoasters use. In DF terms, this would be a windmill, waterwheel, or even some kind of turny-spinny thing turned by animals or Dwarves(I forget what those are called) powering a system of ropes or more likely chains connected to hooks on the carts.
A good example of this system that has been used a lot in reality would be to use a system of chains(or ropes) and winches to pull carts up an incline, then letting gravity take them back down.
Say you're building some kind of mega-construction on the surface, but your (stone of choice) deposits are deep underground. You set up a waterwheel 'power-station' on the surface, and connect it to a line of minecarts. System pulls carts loaded with stone to surface, and gravity takes carts loaded with food and booze down to the miners.
This would also allow a player to use minecarts to move large amounts of material long distances without relying on Dwarf-power, animal power, or dangerous(and sometimes impractical) gravity-based setups.
All of that said, when I first read the DevBlog mentioning minecarts, my mind was filled with 'OhmygodOhmygodOhmygodOhmygod', in the Space-Core's voice, of course.
Edit: Langdon beat me to it.