If there's one area where have heard a lot of complaints on it's the WiFi function on Windows Phone.
The problem is a bit niche but it has frustrated quite a few of you ever since 2010. That "problem" is when the phone goes to the Lock screen, WiFi automatically disconnects. Obviously Microsoft had good intentions in mind here with power-saving as the chief concern. But recently, evidence and personal anecdotes seem to betray the idea that turning off WiFi (when not in use) saves battery.