Larger screens and buttons, sturdier design, maybe better speakers. None are necessities, but all are marketable in one way or another. Of course, Nintendo could add any number of extras (not saying they would, just that they could). An SD card slot or internal memory for music and video files?
See, like I said, Nintendo already has your money so they aren't necessarily targeting you with a redesign. Nintendo wants the PSP owner or the person with neither a PSP or DS to get a DS. If a redesign can convince them to get one, that's a win for Nintendo.