I really like the theme and ambiance but wanted to reiterate what some other reviews have noted. The first place you can possibly die is on the first plant, so if you die there it would be great to start in the room before the plant vs going back to the title screen and watching the tutorial text again. Every elevator could be a checkpoint from then on.
Also to help new players who might not have picked up everything in the tutorial, when you first grab ammo, it could prompt you to "Press A to Reload" - my first time through I didn't think to load my gun and was killed while trying to shoot the plant. When you do reach the first plant, it could also prompt you to "Hold X to AIM, Press C to Shoot", so in the moment people can be reminded. This would make a big difference in getting people into the game and ongoing checkpoints would make progress less frustrating.