In my project I have textViews with associated input keyboard which has the strange look in landscape mode on iPhone 6 Plus under iOS8 as shown below:
The very important thing to do while migrating projects from xCode5.1 (and earlier) to xCode6
is to modify Info.plist file by adding Launch screen interface file base name key

You can event set it to empty string (usually it is a name of the Launch xib). Without this simply line one will have all the problems presented at screenshots above.
The absence of the Launch screen interface file base name results in working in compatibility mode with standard resolution of older devices. Let's run the following code in the project in each case
NSLog(@"bounds = %@", NSStringFromCGRect(UIScreen.mainScreen.bounds));
With Launch xib one gets
bounds = {{0, 0}, {414, 736}} // for iPhone 6Plus
bounds = {{0, 0}, {375, 667}} // for iPhone 6
Without it one gets
bounds = {{0, 0}, {320, 568}}
So, the last case indeed is the compatibility mode.