As a former owner of the 27" 2011 iMac ill throw in my "two cents" on some of the obvious features.
(1) Its thinner - Not necessarily a good thing, once I set mine up I never moved it so the lightness isn't important, the only real positive to this is it looks cooler, which is ironic as the negative is heat, something that was an issue for older iMacs with decent GPU's/CPU's, time shall tell if the new thiness proves to be a mistake.
(2) Anti reflective screen coating - A definite improvement over the mirror of light that the old screen was, however lets be honest here, that was an issue entirely of Apples own making due to their glossy coating. Regardless this is a definitive plus, (NB: It would be nice if Apple brought out new upgrade screen covers for the older iMacs with the new coating, but its unlikely).
(3) Optical drive gone - Won't be missed, anyone that does can grab a cheap USB one for peanuts, I can count on one hand how many times I used the one in my iMac (and I'm not just saying that because I only have one hand!). Ironically aside from burning the occasional audio CD for the car (new radio has USB port) I only used the optical drive to install Windows via Boot camp lol.
(4) Ivy Bridge - Good but nothing to get excited about, I doubt any real world performance between a 3.4ghz SB and a 3.4Ghz IB will be that spectacular, having said that free performance is free. But TBH im much more interested in IB bringing USB3 by default.
(5) USB3 - A nice bonus, however its somewhat ironic that the main thing old iMac users wanted it for (Adding an SSD without having to open the mac or go Thunderbolt) has been sort of brushed aside by Apple now offering a SSD option for the iMac (that doesn't struggle to outperform a Velociraptor like the old 512GB one).
(6) New SSD - Awesome, I can't explain how much I longed for an SSD in my iMac, but the factory one was garbage and to add one to the 2011 I would have had to either open it up or blow £150+ on a TB cable and caddy. Now Apple doesn't just offer a decent one as a factory upgrade but also offers an advanced drive management system that keeps the files that need to be on the SSD on the SSD (without duplicating them like caching).
(7) New GPU's - Great, noticeable performance boosts (the 680MX supposedly being over 2x faster than the old 6970M from the 2011 in 3dMark11) hopefully the new cards won't bring in heat issues. The is still however the problem that the iMacs gaming performance isn't hurt so much by its mobile GPU's but by OSX's archaic OpenGL implementation and its lack of Direct X (obviously the second one isn't easily fixable without Bootcamp or virtual machines).
All in all I like the look of the new systems, I won't be considering going back to Apple due to the OSX constraints but if I could justify blowing that much money on an iMac just to boot camp it then id be after one of these in a heartbeat, awesome machines.