New Mutants could have been excellent. The core characters (SunSpot, Magic, Wolfsbane, Moonstar, and Cannonball) are great, the comic was great, and the concept of teenager heroes dealing with teen life AND super powers seems loaded with story-telling opportunities. Unfortunately, the production seems to run away from those opportunities and is afraid to dig in.
The film fell into the pothole of needing to 'control everything' so much that not only did the lead villain put everyone inside immobilizing force fields, but the entire story was squeezed into a 'psych ward / rehab' style facility instead of being allowed to move around naturally.
There were surprises, but the writing in general was lacking. These comic heroes have depth, but none of it was explored in the movie. From Cannonball & Sunspot not knowing how to use their powers being downplayed, to Moonstar's lack of knowledge about her powers being magnified to overlay the entire film, there was a lack of critical decision making to fully flesh out characters. A burn scar is not background unless you dig into the story of it.
Also, the characters were not treated fairly or well -- couldn't Magic have left whenever she wanted? Why didn't Sunspot use his powers more effectively/often? Who was the doctor watching them? Who is Essex corp?
Instead of exploring subplots in the story, the film feeds tidbits and then hides from all of them. It was like a series of constant misfires.
Last critique -- altho I have more gripes - is that the entire film was supposed to be a horror film, and yet it shies away from delivering any real horror. Cutting away from each scary scene, not using any real horror film technique, and the scariest creatures (Magic's fears) are never explained, easily dispatched, and essentially blind scary-faced fodder. Moonstar's power is interesting, but when you go 100% CG, no one has any emotional investment in what they see. So it was one of those, "Oh, this is the big reveal. Meh." moments.
So it could have been so much more. The actors were ready to deliver, but the writers and the director didn't seem to be able to provide a quality movie for the actors to work with.