I recently looked back at my LD58 game and its reviews. I now know what I did wrong.

1. Use of high-quality sprites for stronger enemies (which MASSIVELY impacted performance and accounted for ~70% of the game's size), while using 2D 16-bit Wolfenstein3D-like textures for others

2. No clear death screen - when players died, the game just had a brown screen and everyone was very confused as to what happened.

3. Lack of atmosphere - the only sounds were weapon pickup and shooting sound. This won't repeat, ambient music is also needed.

4. Grindiness - the chance of bosses that dropped the required parts were very small and it took a very long time to find them and kill them.