Right now, the Steam Music beta is pretty limited. It can only be used in Big Picture mode and can't play every file type (FLAC being notable, but there is an active pole right now to decide which types are the most in-demand.) As far as I understand, it will also only play music locally, but I've yet to attempt to set up a Network File Share so I can't be sure. I also assume that it's accessible on every platform because the notes say that it can be used only in "Big Picture Mode and SteamOS."
|The Media player|
Overall, this only proves that Valve is serious about overtaking platforms dominated by iTunes, Amazon, Netflix, and the like. It seems like every company is on the prowl for the living room. A cloud music/movie collection might be the next step for Steam, or an easy and quick way to setup network shares. Steamboxes are meant to live in harmony with powerful desktops. I'm not super excited for this shift, but I'm hopeful that there won't be a shift at all. I think Steam, while attempting to move into the living room, won't soon forget their core audience and I'd really like to see these moves be more accommodating to them than anything. Edit: I forgot to mention that Steam is planning a desktop utility, so we gamers aren't forgotten.