iOS 8 implements new features in iCloud which will allow for more storage and sharing options of your documents and photos. The problem is that these new features are not compatible with the existing iCloud. Once Apple releases its next OS X for Macs, code named Yosemite, it will all be compatible. This may be in October.
Bottom line: if you get a new iPhone 6 or update your existing phone to iOS 8 do not turn on the option for iCloud Drive in Settings.