When I selected one of my worlds in the Select World screen, I logged straight into the world. What usually happens is I click the world once and all the buttons highlight at the bottom of the screen, and then I click once more and I join the world. This time there was no highlighting of the buttons, and thus no way to delete the world.

Steps to Reproduce:
1. Join Minecraft 14w17a
2. Select Singleplayer
3. Click on any world