Hopefully the aluminum is thinner and then the heat will radiate out easier. There are going to be less moving parts with no optical drive or hdd, so that may generate a bit less heat.
thicker aluminum may have spread the heat faster across the whole back and resulted to better radiant cooling.
All those intakes along the bottom edge may mean mo' better air flow in this revision. I'm just glad none of that air will flow between the screen and glass and deposit dust or smoke.
The vents on the bottom will dissipate the heat just as easily as the vents on the sides of the rMBP. I am guessing because of the size and the ability to access the memory on the 27" model there are vented slots on the back right under the hinge for the base.
I have this baby in use for a couple of months now and it doesn't seem to get all that hot. You can hear the fan spinning a bit but that's it. So I think they handled that part very well.