I loved that the 3DS supports mp3 playback via 3DS Sound but I think that it should be able to play in the background so you can listen to your favorite songs while browsing the internet or playing your favorite game. I for one would love to spice up a Pokémon battle with some rock n roll.
Until I can find a quote thats really deep on a spiritual level, I'll just put this here.
After looking it up, apparently the PSP did not have it unless one installed homebrew software. Can't put the two on even footing in that case. Obviously the PSP was more multimedia-friendly than the 3DS a generation prior, so the early emphasis was evident. Nintendo's focus on games and less the all-in-one gimmick of course garnered greater success.