When attempting to log into Minecraft with 1.9 set, it would crash repeatedly. I checked the error message, and it seemed to have to do with issues loading the world graphics, so I fiddled with the video settings until I found that it was Smooth Lighting on that crashes the game when attempting to start it, as it launches with it off.

Many are having this issue, as Smooth Lighting is usually on by default, so to reproduce, simply
1. Turn on Smooth Lighting and
2. Attempt to launch a single player game