Roku is (will be? not sure if this rolled out yet) sharing any TV Everywhere login, so at least you only need to log in to your cable provider once. TVE is used by the broadcast and cable networks, like NBC, USA, SyFy, HBO, etc. Unfortunately this "single sign on" will not apply to Netflix, Amazon, or Hulu.
I know Roku can search across all channels, I'm not sure if it's possible to create a playlist across them.