The GBA is bascially a handheld SNES.
Exactly, minus 2 buttons and like 6 PCM channels. And the fact that 32MHZ is a little faster than 21MHZ...
I think the biggest difference is the fact that cart sizes have shot through the roof. Even a smaller GBA game would have been a huge SNES game. This naturally makes for more impressive stuff.
I haven't noticed the lower number of PCM channels, but the audio on GBA is really bad. Its all raspy and hissy and stuff. This noise really messes up games like FFVI, and Yoshi's Island, which both have really great music on the SNES. The GBA for the most part pulls off these tunes with no problem, other than the noise.