The mouse. It sucks. The latency too (over long thin pipes). I prefer to live in a textual universe anyways. I can't not use a GUI browser for a lot of things, sadly, and occasionally I end up using an IDE, but as much as possible I use tmux/screen + $EDITOR + cscope + scripts + ... as my UI/IDE. It's just better. It works great over mosh, over SSH, over long thin pipes, over short wide pipes -- it just works. Latency is low, and mouse use nil -- win-win.

EDIT: Also, it's a lot easier to organize and find my way around nested tmuxes than it is to organize tons of windows. Yes: I nest tmuxes.