A ‘Don’t Suck’ Button is a setting, turned off by default and usually hidden in a corner of the UI or configuration file, which fixes a common issue that new users experience, and/or turns on the behavior that everyone wants from the product in the first place.
The best examples of ‘Don’t Suck’ Buttons involve settings with no obvious disadvantage to turning them on. They’re just turned off by default and the software makes you go through extra effort to tell it “No, I don’t want you to suck.”
Usually these originated from some kind of automation, new protocol or new feature that started out as experimental and was locked behind a setting, then became universally relied upon but somehow never enabled by default.
I used to have a list of examples of these. One that comes to mind from my TV: “Allow HDMI Devices to Control this TV”. If you have an Apple TV or something, it won’t change the volume or be able to turn the TV on or off automatically unless you know enough to go into settings and tell the TV “Don’t Suck”.