Mojang warns that sometime "in the far future" Minecraft will require newer version of OpenGL. This means that those of us playing on a Mac, or a really old computer, might not be able to update. They're warning us now, but say that this is a long time coming.
In the far future (at least past Minecraft version 1.8, if not later), Minecraft will no longer support OpenGL versions older than 2.1.
Why are you removing support for older OpenGL versions?
OpenGL 2.1 was released in 2006, and is not found in graphics cards that are sold today. Most new games require OpenGL versions 3 and higher. Making this change will help drastically increase performance of the game, and supporting versions 2.1 and higher is also a requirement for a part of the upcoming Minecraft plugin API.
What is OpenGL?
OpenGL is a programming interface for rendering graphics. You can find out more about OpenGL on Wikipedia.
How can I tell if my version of OpenGL is too old?
If your version of OpenGL is too old, and you're running snapshot version 13w38b, or a release version newer than 1.6.4, you will see the following title screen:
What if I don't upgrade past OpenGL 2.1?
If you elect not to upgrade, there will eventually be a time where you will no longer be able to update Minecraft to a newer version, and will only be able to play versions that came before the change.