The only thing that bugs me about neovim is that it seems to require workarounds to get the escape button to behave the same as in vim. When I press escape to exit insert mode, there is a small delay. If I press a key (e.g. 'j' to go down) too quickly after Escape, it inserts the 'j' and cancels what escape is meant to do.
In tmux I know of a workaround (iirc, 'set escape-delay 0' or something similar), but I wish I didn't have to change a tmux setting so one program can function. It's really annoying.. but ultimately worth it, I guess.
That said — to the neovim team: congratulations on reaching the 2.0 milestone!