It's a million little things for me. Apple Music is label and album centric, Spotify is social playlist centric. I prefer Spotify's UI/UX vastly over the Music app. I prefer Spotify's recommendation algorithms. And yes, Spotify Connect is kind of awesome (although not that different than AirPlay, really). Many of the artists I follow who no longer release tracks in an album format will release new tracks and remixes to Spotify and YouTube first, and Apple Music later, if at all.
None of this may matter to other people, and I don't presume to speak for anyone but myself. The one thing about Apple Music that does make me jealous is Apple Music Classical—Spotify is... challenging to use for that type of music, absolutely. But for that, at least, I can fall back to my ripped collection. It's not enough to make me want to switch.