But I will try to put down all the reasons calmly. Keep in mind, this is just one man's opinion.
First off, the controls. Using a d-pad to navigate a 3-dimensional space feels very awkward. 8 directions is not enough for me to feel in control. My ability to control how fast I move is also lacking. When shooting at anything besides enemies, the only way to get any semblance of accuracy is to use first-person mode. Switching views is relatively quick, but being forced to do it a lot is irritating.
Fighting is alright. It's easy to dodge enemies (basically just spam random directions on the d-pad), and it feels pretty badass while you're doing it, and your shots seem to automatically lock onto enemies. The problem is that it takes forever. The enemies have way too much HP, and there's way too many of them. Maybe it's just that I got bored doing almost the exact same thing so many times, but I could swear that it took ten minutes to get through some rooms, and the entire time was just dodging and shooting. It gets a little more exciting with larger enemies that I can do all the fancy melee combat to, but those are more the exception than the rule.
Battling in first-person mode, by the way, I found to be the single most annoying thing I've ever been asked to do in a video game.
The story is, like with many people, the biggest problem for me. It makes Samus trusting the Federation in any capacity in Fusion seem incomprehensible. Samus is just as introspective as in Fusion, but her trauma and fear feels unnatural and not in agreement with past characterization. If you've seen the Ridley scene, you know what I'm talking about. Martin's voice acting doesn't help. Neither do the constant monologues. Especially not when combined.
The explanation of Samus's inability to use her items is dumb. It started off making enough sense, with the whole "don't use power bombs because it would melt us" thing, but when she's miles away from any of her teammates and getting killed by a giant monster, it starts to fall apart, especially with items like the Varia Suit (which makes one wonder, why wouldn't Samus always have that on?). The worst part is that this stupid concept is brought up throughout the game, so you can never forget about it. Having a reason for Samus lose access to her equipment is a nice idea, but it was done better in Fusion.