Besides confirming both the Joker and Green Lantern as fighters, this video of Mortal Kombat Vs. DC footage made the SDCC rounds, where it revealed...nothing. Didn't even show Green Lantern or Joker in action. So why am I posting it? No idea. Just seems like the right thing to do.
It does show off some of the gameplay mechanics, which are terrible because it's a Mortal Kombat game. It also shows the cool Superman pushing Scorpion through a city block or two move, which I'm still confident any character can do, including Catwoman. It also appears that Batman can only throw his Batarangs at a 35 degree angle, over the head of his opponents. And the Flash—well, he looks all right. Oh, Kotaku has a pic of what the Joker looks like, if you're interested.