Now originally I thought it was only the people who bought the game during alpha that would only have to pay once, but I may be wrong. But I think I may be wrong as I have just seen here on the website that you also get the full version when it is released.
"Please note that when you buy the game, you're paying for the game as it is right now!
You will also get the full version of Minecraft when it is released. Read the copyright information for more information on your purchase."
You get the all updates and the Full version of the game. What you do NOT get for free are possible expansion/booster packs which may or may not happen in the future. However there aren't any games that I can think of that give expansion/booster packs for free so as for what you get is basically the game and it's updates, If large expansion packs come along they may cost a couple bucks to get, but they won't be necessary to continue playing the game.
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