Even if Resident Evil Zero doesn't show any improved changes, it's still a great game.
Graphics are really nice and beautiful.
Sounds are great. Creepy sounds from a zombie around the corner to eerie BGM. I really like the BGM in Zero, it has one of the best music in the series. Well in my opinion.
The ability to play as two characters and not being alone in certain rooms is pretty nice idea, but it's not fun when the other characters starts waisting bullets and you end up with out ammo, then you can say good bye if you play Normal or Hard mode. Also the ability to drop the items anywhere is pretty nice addition, but it gets frustrating finding them and dragging them one by one if you have too many items you want/need to take.
If you beat the game you get this minigame called "Leech Hunter", where you collect these leech objects as many as possible, beating this minigame unlocks more weapons/ammo to the Normal game, it's similar to The Mercenaries Game in Resident Evil 3.
You can say that the gameplay of Resident Evil Zero isn't improved much, it's still a fun game to play.