I love the design of the main character, the UI looks gorgeous, the music fuckin' rocks, and it feels so good once you get shmovin' with all the equipment upgrades. I also really like that you can throw temp. weapons to save on ammo.
Only somewhat negative thing I have is that I didn't really vibe with the Shock Gun and never really used it, but that wasn't a big deal. That, and I kept flickering my flashlight when trying to throw my weapon. I changed it to R, which was easier being closer to my index, but if the full release has reloading weapons with R that might be tricky.
Those are frankly really small nitpicks because this demo was great, it feels like Metroid from a different timeline.