What problems on the previous 2012-2013 iMac are you talking? I have one next to the new full spec i7/4Ghz retina iMac and I can assure you, that the machine is whisper quite in any games, while retina does come on with airplane-like fans going off in 2-3min in the game - and I am only at the menu screen. Not a heavy gamer, but I certainly do not expect newer system be more noisier then previous gen. This issue is also worrying in the long term. Anyway, I made my mind and unfortunately returning this iMac back to Apple. The screen is absolute stunner, but not worth taking the risk, I rather wait regardless how tempting this is. Its very expansive machine and as someone said before - I expect nothing but the best from apple, and this is - after having more then 25 mac in my 25 years of history with Apple - not there yet.
I also found, that on my machine the screen is nor properly fitted in the bottom where it meets the aluminium with Apple logo. It's kind of wave and looks like its not glued correctly. Still comparing my with my previous iMac from 2012 and the screen is perfectly fitted in the bottom. You see only slight aluminium edge, on the retina it disappears under the bulge of the screen on few spots.
I would recommend anyone to check macroumors for thread about throttling and heat issues before pulling the plug. I am not happy about it, because I was waiting for this machine for a long time, but I would recommend to do DD before pulling the cash out of the pocket.