Movement: Would be good to feel a bit more weight in movement.
Grenade: Would be cool if it can be shoot to explode early.
Cooldown indication: Might be better to have something like a progress bar to see cooldown moving instead of just simple red/blue.
Tutorial have too much info but still don't tell the player what the items and skills actually do, might as well skip this part and introduce them as the game progress with a bit more details.
Item Shield need clearer boundary.
Tooltip over items, skills and weapons would be helpful.
Loadout should persist between levels.