To me, Spotify's killer feature is Spotify connect and official Linux support.
I keep looking at Deezer for its cheap hi-def support (the premium plan now has FLAC, for the same price as Spotify). But integrations with non-officially sanctioned devices seem janky, at best.
Maybe things have improved, but at one point I had a Yamaha receiver, officially supported, which basically replicated the Deezer interface inside the receiver's app. I couldn't control it from my computer. IIRC Tidal was the same. With Spotify, this just works. Bonus points for supporting the use case of "having friends over who can play their music on my stereo".
I also use multiple "devices" to play music: phone, personal laptop, work laptop, personal desktops, media PC. I don't know how Deezer's "three registered device limit" is implemented, but I'd hate to have to log in again every time I change the computer.