#1 - July 5, 2018, 6:39 a.m.
1) Computers don't devote resources the same way to windowed programs as they do fullscreen programs
2) Computers can alter their behavior when running a fullscreen program. They can turn off notifications, put antivirus into a low resource mode, adjust heat management etc.
3) Gsync users will be forced to run gsync on even on their desktop and turn vsync off globally instead of being able to adjust it in game
4) There are reasons every other AAA developer not only supports fullscreen but enables it by default. A AAA game should be forced to run like a shady free to play browser based game
5) Taking choice away is ALWAYS a bad thing. I know that I like to take direct control of my system and tweak my performance. I bought a PC because I want that level of control. I want options.
6) Color profiles. Many people set up their own custom color profiles, most of these rely of full screen to work properly.
Blizz, if you see this, please, please dont let this change go live.
EDIT: Here is a source confirming this change is intended for those who are too shocked to believe a first party developer would do this on purpose
Credit to Charsi for finding this