I'm surprised the lack of .mp3s is a big deal for some people, though I guess it's kinda similar to the outcry over Xbox One essentially going digital only (which I participated in).
For me, personally, I now rely entirely on cloud services for music playback. It used to be Last.fm radio, but now I've uploaded all my old CDs to Google Music, and I also sub to Google Music All Access. If I fear I won't have access to an internet connection or I don't want to use up my data, the Google Music app lets me "pin" any music I want (including All Access content) to whatever device I'm using, storing it locally.
I'd imagine Sony was expecting most people to use apps like this and their own Music Unlimited as opposed to .mp3s, so they didn't bother including .mp3 support given they expected no one would care.
DLNA support, on the other hand, I can easily understand. Though the lack of .mkv support made that feature pretty much useless for me too.
Edit: If they ink a deal with Google for Google TV/Play features (music, movie, book, etc. access) I will be SO stoked. But it seems Google doesn't let people do that too often, looking at the current slate of media boxes and consoles.