I have to agree, the first one does look great. While I do see some improvements, I had to go look at shots of the original to notice them. The characters have better detail, most notably enemies. More curved/less blocky edges in general on plant life and enemies. Better lighting in a few places. Hard to imagine they could amount to enough to make the game any more fun but you never know.
One thing that really impressed me about the graphics of the first one (other than being very clean) was at times how many objects could be moving around on screen and yet still maintain such fluid animation.