In my experience there hasn't been a game that has been seriously hindered by the lack of buttons on the GBA. Perhaps my memory is hazy but I have a hard time remember any games for the SNES that used all the buttons. There were typically 1 or 2 buttons that were assigned the same function so the loss of 2 buttons is negligible.[/QUOTE]
I think that's because there haven't been as many SNES ports on the GBA as most people tend to think.
I think most SNES games probably had about three buttons that you could consider "indespensable". For Zelda LTTP, Mario World, and Yoshi's Island, that was the case, so Nintendo mapped the third buttons to the "R" trigger, which was only moderately annoying.
AFAIK, SNES-ports on the GBA were outweighed by NES-ports, which (surprise) only needed two buttons.
IMO, like the GBA, the DS isn't going to get the flood of N64-ports everyone is expecting (except for a small handful of them coming mostly from Nintendo), and it'll get outweighed by SNES-ports.