This post will be updated in sync with Minecraft Forum's FAQ Thread. Please do not ask for support in this post; use #minecrafthelp IRC Chat for that.

You can find the changelog for 1.12 here.

1. Downgrading

As always, downgrading is not, was never and will never be officially supported! Please do not run 1.12 worlds in older versions of Minecraft.

2. Known Incompatible Software

Some software is known to break Minecraft. If your Minecraft stops working, check this list of known incompatible software and see if you have any of those installed. If you do, those software will need to be uninstalled.

One particular offender is ByteFence. If you are seeing an error message like this, it's most likely ByteFence which should be removed from your system. (Bug Report Here)

Error details: Attempting to move successful download to C:/Users\User\AppData\Roaming\.minecraft\assets\objects\somethingsomethingsomething

The process cannot access the file because it is being used by another process

3. System Requirements

Starting from 1.12, Minecraft will no longer support Java 6 or 7. If you are using the latest launcher, you do not need to worry about this as Minecraft ships with its own version of Java. If you know you are using your own version of Java however, you'll need to make sure you're using Java 8.

You can get Java 8 for all supported platforms here.

The OpenGL requirement for 1.12 stays at OpenGL 1.3 (according to internal sources). Please report back if you have a OpenGL 1.3 only system that refuses to run Minecraft even at very low graphics settings.

Hardware requirements for 1.12 is currently staying the same as 1.11, but this may change in the future.

4. Graphics Glitching For No Reason After Upgrading

If your graphics suddenly started glitching since upgrading to 1.12, chances are that your graphics drivers are out of date.

Make sure you have the latest drivers installed and set VBO and VSync to On and set Mipmap to 0 in Video Settings (Options -> Video Settings -> Use VSync/Use VBO -> On).

If you are on a laptop with switchable graphics, you may also want to check that your computer is indeed using the dedicated graphics card.

Nvidia Optimus Instructions

AMD Switchable Graphics Instructions

5. /give and /replaceitem Behaviour Change

Since 1.12, /give and /replaceitem no longer bypass item-stacking restrictions and will fail if you attempt to do so. This is to prevent performance issues and bugs. Bug report here