I'm not familiar with neovim's terminal emulator. What happens when I press CtrlX-CtrlE? Is it similar to a normal terminal with readline set to vi binding?
It's quite possible that we'll see those features in the future, what with neovim's separate frontend vs. backend architecture.
Immediate deal-killer.
If your workflow involves a) more than one computer and b) usage of the terminal (surely describing most developers), then tmux attach/detach is easily one of the most awesome things since vim itself.
It's worth noting that I'm using other means to keep my state synced between my working environments (place) or between working sessions (time). Version control is part of that (such as private git branches), good environment automation is part (via Vagrant + provisioning, packagers like npm, etc.), and automated vim session save/restore is another part of that. For example, if a vim dies or if I quit, my entire session state is completely restored for that project when I start vim in that directory again. That all provides quite a lot of what I'd traditionally depend on tmux for.