Looking at the pics, I really don't like how the top half of the unit is raised around the edges, making it concave, presumably to seal it around the edges when it's closed. That sort of thing makes it bothersome to easily open and close the unit. The rounded edges of the GBA SP are much nicer.
They took one of the problems of the DS's design (being flat around the edges), and made it even worse. While quite obviously looking at a GBA SP's design, which had the solution.
Nintendo probably thought they needed to do this because they didn't recess the D-pad and buttons (which is a great thing, IMO), but if you just made two recessed circles where the buttons meet the top half, then you're fine. As-is, all they did is just recess the entire top half of the unit, which left them unable to round the corners.