Sounds 50/50 so far - on the one hand I feel that it might a closed system much like the Chromebook except with Steam Workshop support which is how they'll get around the 'well, if it's closed, then no modding?!' question. This part about 'Were working with many of the media services you know and love. Soon we will begin bringing them online' seems to imply that they won't allow users to install their own software on top of SteamOS, but they'll provide basic services to these. Maybe that includes streaming services.
Moreover the 'Parental Controls'/family territory section does seem to point towards some locking down of services to prevent kids going around Steam to play Saints Row IV.
Of course, the other hand says "Well, that's speculation thus far off little to squat information".