You've done a very good job of taking something with over 60 years of history and making something new and original. Your style in not only unique, but it breathes new life into well established characters and brings a new take on them. Way to make these characters your own while still respecting the existing history. I would love to see something like this as an animated TV show produced by Warner Bros. Unlike the comics work of the past where everyone had to draw characters essential similar, not the same, but similar, the new generation of comic creators have begun to stretch and expand on what people's pre-conceived notion of what comic art should look like. Your work is a very clear example of that.
I think this is very good, like mikegagnon said, you took something historic and made it your own.
The only things that I could say is that Batman's left arm blends in with his chest, making it hard to distinguish arm from chest. And on a personal opinion thing, Superman's cape, should maybe rest higher on his shoulder, going down so far seems awkward.
I especially like your Nightwing, he feels youthful but serious at the same time.
Superman is good, especially the look on his face. Can't say I like Darkseid that much, and Mr. Freeze is pretty standard. Batman looks pretty cool, and I like how his cape covers up the top of his insignia.
But Nightwing? Oh MAN. That is one sexy design. Great insignia, great belt, great pose, great use of split-toe ninja boots on the feet, sweet haircut.. I would have sex with that design. Maybe not really, but DANG. Awesome. You should make more of him.
Only critique would be that Batman should be wearing actual boots with tread to look right, and not the usual sock-like booties that other superheroes seem to be shown with. I've never understood that whole sock thing and wish artists would show the characters with a real boot or shoe.