Overall great game with an interesting story but not without its flaws.
First I have to say that i played on easy and I haven't completed the demo yet as am just at the second boss and i don't think I will continue playing it since i am a casual player and having to repeat all the second part because i died on the boss made me lose interest in the game.
Maybe adding a save before the boss would make me continue playing.
here's some ideas :
-a special health slot that automatically fill with a health item if you have one.
-the ability to combine two things that are the same type into one but every inventory slot can only handle a certain number of items for example you can combine two plastic ballistics containing each 5 into one containing 10 or two plastic ballistics one contains 7 and the other 5 and when you combine them you get one with 10 and the other with only 2 this would highly decrease junk items
-a use button with the move/discard/drop would be nice
-instead of a yes or no question every time you discard something show an undo button for several seconds
edit :
i retried my luck yesterday before i read your answer and i missed the save spot again :(
i believe that something as important as saving should be obvious to the eye, perhaps a brighter color for the door or an arrow would help.
Then lets talk about for the second boss, i choose the easy difficulty because am more interested in the story than the challenge and the first boss was fairly easy for me but the second one ? i don't believe i could beat him so maybe a tip from you can help.
And finally for the use button idea :
i like your last idea since entering and exiting the inventory already take a turn it would make sense