Mojang will be at PAX. We'll have the first playable version of the Adventure Update there, and it will be released to the public once we've adressed the feedback we get at PAX.

Yes, this is a marketing thing. ;) But also, we're doing some pretty bold changes to the game, and knowing we have the time to just play around with features we might now want to release is pretty liberating. Combat in 1.8, for example, is already way more fun than in 1.7, but it need further tweaking.
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