Your post makes sense. Level design and puzzles are important too, but not AS important as storyline (in this case), because it's only the GBA. If you're playing a GBA game, you can't expect stunning graphics, so there has to be SOME incentive.
Only in Gen V did Game Freak finally start to make improvement on their graphics -- they (and Nintendo) are still way behind.
But omg if you've played Mario Kart 7 on the 3DS, Nintendo is catching up fast with graphics. For a handheld, it's amazing. And the courses are fun too.