My scattershot thoughts:
* I like B-movies, and I like schlock. I felt this one failed for me because it wound up being somewhere kind of in the middle where it didn't 100% want to commit to being a B-flick, so you wound up slowly thinking this is a bad flick, this a worse flick, this is... oh... it's trying to do this. And then right when you kind of get on board with that, the third act almost shifts completely away from horror to a matrix/ninja style affair.
* Wan borrowed heavily from Lights Out (which he produced) to the point where the two movies could have had entire segments swapped out with one another.
* But the obvious inspiration is old Argento films. I mean hell, just compare the promo poster for Malignant to Tenebrae or something like Stendhal Syndrome. Very obvious what he was going for. The film style (and music to an extent) was very much reminiscent of Argento's "lets do this really weird camera angle in this scene for no apparent reason". When I'm watching 70's and 80's Italian horror it works because... well it's the 70's and 80's and that's what I've come to expect from that genre. It seems/seemed out of place in an a modern film.
* I think part of my letdown is what I was hoping for a good James Wan horror movie. Conjuring, Conjuring 2 and Insidious are some of the best horror in the last few decades. I'm not a huge Saw fan, but there's no denying it's success (I believe one of the highest grossing franchises of all time?). Even in it's B-movieness, there were no legitimate scares in this film.
* Even knowing it's not meant to be all that serious, a movie still needs to stay true to the world it creates. After the reveal of what we were dealing with, many of the scenes from earlier in the film don't actually mesh... eg without spoilers - why would the couch cushion slowly rise up in the opening act?
* The story itself also just borrowed a little too heavily from other books/films that simply did it better. Stephen King's 'The Dark Half' was basically the same premise, and while his film adaptations certainly are more crap than good, you can actually make a case that the '93 film was actually a far better film from both a competent standpoint but even from a B-movie-esque type standpoint. It's pieces fit, while I don't feel like Malignant's did.
But as someone mentioned in an earlier post, I think ultimately what we had here was a studio that wanted a horror movie for September and Wan quickly threw one together for them doing something from an idea he had wanted to explore but then molding it into a giallo-type film -- not something he necessarily thought would be a commercial success. It's why you see the marketing and the trailer very much try to conceal the type of movie it really is.