Reply from Samsung Support just arrived telling that it's the Spotify developers responsibility to make Spotify available in Smart Hub. Come on developers, I can't wait to listen to Spotify in my living room, especially when I have guests!
The new samsung uhd use tizen (based on linux) as OS for their smart tv.
This means programns need to be recompiled for this OS. You have now multiple competiting OS for the new SmartTVs offered formthe different manufacturer: IIRC, WebOS for LG, FirefoxOS for Panasonic, and Tizen for Samsung.
Tizen runs on smartwatches and alternative phones.
Since they all provides SDKs, I think it is matter of time. Other interesting programs like Plex are missing as well.
From Spotify I'd expect having it ready even before the release. Tizen OS was announced almost a year ago at the Tizen Developer Conference, time more then enough for testing.
About Plex: beta could be installed from USB and the stable version should appear in the Smart Hub this week according to their developers. I don't understand why Spotify is not releasing a downloadable beta (if they have any) for all of us, keeping us in the dark and frustrated.