Enemies should glow green when hit with acid. It seems so obvious, but we didn't think about it until Doug was watching me play recently and he noticed that I couldn't tell whether or not I had hit the enemies with acid. It was a small change but it made a large and satisfying impact on the game.
This is one of our 'duh' moments where we wonder why we didn't think of it sooner. It is one of those situations where working harder or faster wouldn't have made a difference, instead we just had to wait for a spark to trigger the idea.