After joining a singleplayer-world created in Minecraft 1.8 and pressing the "Escape"-Key on my keyboard, i realised the width "Difficulty"-Button in the gamemenu is not adjusted to have the same width as all the other buttons and control-elements (see attached screenshot).
Note: This is only a minor beauty-bug, not gamebreaking or anything.
That is your texture pack not correctly updated to 1.8.
Next to the difficulty button SHOULD be a 'Lock' icon which is a new feature in version 1.8 which allows you to lock a world's difficulty.
If you switch back to the default texture pack you should be able to see it.