As Lat already mentioned, the main problem with your level is the lack of indication of where to go and what to do. Either you have to step on a piece of floor with a different flat texture, but it's obscured by the brightness, or you have to defeat all the enemies to proceed but it's simply not clear that you have to.
The worst offender would be the No More Enemies room itself - it's way too cramped, it's got a confusing layout, both chosen enemy types are slow and tend to lurk behind crates where it's difficult to see them while the camera object has the awful tendency to get stuck between those spaces, worsening the situation. Lessening the number of enemies, choosing a faster, flying enemy type, or making the enemies hard to reach but not in a way that it tires the player from proceeding would make the room a lot better.
Also Romio already pointed out that if you die after activating the Hot Shelter room Star Post, you respawn inside the door, having no other option than gaming over. Either placing the Star Post a little further or giving the linedef that opens the door a Once trigger should fix that.