My 6 Gets hot too but only when im whatching a 1080p movie or playing a heavy game (basically any task that puts near 100% load on the chip for a while).
guess non we can do about it except hoping for a fix in iOS 8.1
I overheated my phone yesterday (got the temperature warning) while doing a time-lapse of the sunset (so the overheating was to be expected). Phone should be fine though as long as you don't overheat it too often.
Under normal use, my 6 never gets warm at all (though I'm not a very heavy user--no games..only email, social networking, etc.)
iOS 8 now has an option to tell you want app is using most of your battery. If you check that, maybe that could be what is causing your iPhone to heat up. Also, a bad LTE connection can cause your phone to heat up as well.