Apple is going to block a loophole that allowed people to play classic video games and run unapproved apps on iOS 8 without jailbreaking, so if you just started playing Nintendo from your iPhone 6, you're going to want to hold off on updating to iOS 8.1.
People were using something called a "date trick" to install console emulators, where they would move back the date and time at least two months on an iPad or iPhone and then download the emulator on Safari. Before this trick, you had to jailbreak an iOS device to enjoy an emulator.
Losing this loophole will cut down the growing iOS emulator community, since the easy trick meant casual classic gaming fans who don't want to jailbreak their phones were actually able to install emulators. Popular emulator sites like iEmulator, for instance, relied on the date trick.
It's not like Apple is closing this loophole just to be a dick. Nintendo can't be too thrilled about people playing bootlegged versions of SNES on their phones, and allowing emulators could create an issue between the two. Plus, Apple has been criticised for various security failures, and it's their prerogative to close as many holes as possible.
Screenshot via YouTube