DC Universe Online game director Chris Cao and creative director Jens Andersen regularly answer questions about the upcoming game that they receive on Facebook, and in the latest episode of "Inside the Studio" they cover a lot of game mechanics. Like, can you shapeshift in the game? Yes. Can you use magic? Sure. Will there be real-time day/night cycles? Nope, as Gotham and Metropolis have their own persona -- the Bat Signal doesn't work quite as well during the day, after all.
If you're curious about DCUO, it's a nice primer on how some of the more specific elements of the game will actuality work.