I was able to load worlds perfectly only a few days ago in snapshot 13w39b.
Now I'm after going into my world at least 5 times, getting out of the whole game and back in and the world wont load.
When I select my world, once it's loaded straight away it gets out of the game.
Please attach the complete output of the "Development console" tab of the launcher.
Click the text, press CTRL-A to select all, then CTRL-C to copy, and CTRL-V to paste in a comment on this ticket.