Found out that updating from 1.7.4 to the snapshot 14w02c corrupted the world.
Here is a link to the thread: http://www.minecraftforum.net/topic/2293765-snapshot-14w02c-removed-single-player-world/
It's easily fixed with McEdit but it would be nice if the transition from 1.7.4 to 1.8 goes a bit smother and not via McEdit if you get what I mean.
According to that forum thread that crash did happen with 14w02b (Outdated!)
Besides that
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