I'm on the cusp of buying a PS4 -- probably as a family Christmas present. If the Windows 10 update for XBox allows me to play my Steam games on the XBox, it's a game changer for both the XBox and Steam. I'm already slightly invested in the Sony space, and my friends have already upgraded to PS4, so I have some ability to network and share games, which pushes me towards a PS4.
I don't get much on Steam, anymore, because I don't want to play action games on my PC. I play Civilization on my Surface, but otherwise do most of my gaming from the console. With the frequent sales on Steam, I'd probably be prompted to increase my gaming budget and spend more of it on Steam, if I could play them in the living room. I don't want to get a Steam Machine, though, because 1) I don't have faith that the first gen won't feel a bit beta and 2) I still want to be in the "console first" space (i.e. walk into Target and buy a disk, get a used disk, etc.).