... its battery life.
I just upgraded from a Galaxy S3 to an S5. Oh sure, the screen is bigger, the processor is faster, the camera is better and it instantly knows precisely where it is with speed and accuracy unheard of in the S3, but the best part of all is how long it lasts. I don't have any hard data as I've never let it drain to zero, but both my wife and I agree that our S5s best feature is the battery life.
That was unexpected, to be sure.