That this game lacks footsies and a mid-range game(which is false even in the demo, but I guess we're ignoring Nightwing, Catwoman and any other characters with crazy reach)
That zoning characters are suddenly OP because you can't dash cancel, ignoring armor, Flash's trait, Aquaman's shield, air dashes, dive kicks... Yeah. Not to mention background interactables can be thrown and some have invincibility frames in their start-up to boot.
That characters without armor and non-zoners are going to get destroyed... Um. Flash can slow down time. Fast mix-ups(another stupid complaint) for a good part of the cast, too. Pressure has added benefit of Roman Cancels if I remember correctly as well. Armored moves are generally punishable through block, or crouch and whiff punish. So, uh.. derp
Almost none of these complaints are legitimate; it's all because they only play MK(I'm pretty much the same, I have mediocre experience with Skullgirls and Soul Calibur.) and they're throwing wild conclusions out while being so painfully ignorant of all of the mechanics in this game(see above). Dash cancels are really only sort of a problem compared to crappy walk speeds, and even then the biggest problem is still that you don't play other fighting games.
tl;dr Learn what you're talking about and before spouting shit and complaining. It's literally day 1 of a seven month old demo and people are already complaining. Expecting this game to be like MK when NRS has stated and showed that it's trying to take qualities of all games is stupid. GGs though