Notes:

There is a common error that causes the game to not properly launch and gray out the "play" button. To fix this, delete your .minecraft folder.


In order to know if you are affected by the error, a sample error is provided:
------------------------------------------------------------------------------------------
Job 'Version & Libraries' finished successfully
Launching game
Looking for old natives to clean up...
Fatal error launching game. Report this to http://mojang.atlassian.net please!
java.lang.NullPointerException
    at net.minecraft.launcher.GameLauncher.cleanOldNatives(GameLauncher.java:515)
    at net.minecraft.launcher.GameLauncher.launchGame(GameLauncher.java:360)
    at net.minecraft.launcher.GameLauncher.onDownloadJobFinished(GameLauncher.java:693)
    at net.minecraft.launcher.updater.download.DownloadJob.popAndDownload(DownloadJob.java:124)
    at net.minecraft.launcher.updater.download.DownloadJob.access$000(DownloadJob.java:15)
    at net.minecraft.launcher.updater.download.DownloadJob$1.run(DownloadJob.java:94)
    at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
    at java.util.concurrent.FutureTask.run(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.
------------------------------------------------------------------------------------------


.minecraft/minecraft folder locations:

Windows  =	%appdata%\.minecraft
Linux    =	~/.minecraft
Mac OS X =	~/Library/Application Support/minecraft