It used to be that running emacs from, say, an ssh connection would start it in the terminal. It used to be smart enough to see that DISPLAY was not set, so it should not try to open a window. This has been the default behavior for decades. Now, running emacs from an ssh connection will either fail because it can't open a window, or, even worse, it will open a window on a display somewhere... across the room, across the world, who knows!

Reproducible: Always

Steps to Reproduce:
1. ssh into a machine (or otherwise unset DISPLAY).
2. Run emacs

Actual Results:  

Emacs tries to open a new window on the display. This may fail or open a window on a display that is not where the user is seated.

Expected Results:  

Emacs starts in the terminal (as by emacs -nw).