Have been having this same problem (heating up) and cannot pinpoint the app that is hanging up and draining the battery. Have been reading all the messages and have started turning off things one at a time to see if that has any effect.
As I said, there doesn't seem to be any special app that causes it. All of my email accounts are set to Manual, except of course my Microsoft account, which you can't set to manual and there are times when even checking that account, will start the phone heating up. Sometimes, it will be Messaging, sometimes it will be Internet Explorer, or any other browser I have installed. I certainly hope the Official Release will correct any and all of the "bugs" mentioned in all of these messages, otherwise people are really going to be ticked off.
This update is literally the best so I don't want to go back to 8.0. Could it be that the Nokia Lumia 1520's are not capable of running the 8.1 version? I see in some advertising for newer Nokia Lumia phones and ones from HTC, they state "Windows 8.1 update ready". Or, could it be the AT&T configurations that are causing the problems? Don't know, but whatever it is, I hope it gets fixed.