A few friends won't be playing Classic, and quite a few of them are going to PvE realms. I'll be leaving too many behind sadly, and I wish it was another way. As for characters, i've switched mains a few times over the years, i'm still attached to all of them but I won't mind leaving them behind for a while. I'm leaving behind my goal of never missing Ahead of the Curve achievements (killing a Heroic raid end-boss before the next raid comes out), I gave up on Queen Azshara to focus on Classic, researching and making sure I know what i'm doing, and i'll of course miss multiple raid tiers.
I'm leaving behind my love of the constantly evolving changing story, I was still deeply engaged in what was happening in Battle for Azeroth. The story has gotten really funky, but i'm a huge nerd and I still enjoyed the twists and turns for the most part. Wrathion is finally making a return, and it looks like we'll be going to the Dragon Isles in the near future. But oh well, Classic is where my home will be, and i've accepted and embraced that.