Well usually when Apple references upgrades in its software they say things like "the worlds most advanced operating system gets even better". I believe they software could be great on its own with current Macs... and it probably will run great. The absolute need to upgrade the hardware is not there. Its everyone that wants to stay on the forefront of technology that is pushing the hardware. Its the want of the consumer not the absolute need, as is the case for the Mac Pro, from what I understand.
I hope people don't misconstrue that statement. From everything that I have read on this and other forums, the current iMac runs like a dream. Also based off of the leaked geekbench tests the numbers only indicate a slight boost performance, not nearly as drastic as the MBP from what I have read. This could be for a few reasons, in my book. Either the upgrade is only a minor spec bump to appease the masses( which would be un-Apple of them, as they do what they want anyway) or it could be a major overhaul including a retina display, and the display drawing on the upgraded components to allow only a slight bump in scores.. thus giving the image of a slightly spec bumped iMac. I'm fairly new to the technical side of the Apple world but from what I understand.. a Retina display will draw a significant amount of computing power from the computer taking away from other tasks.. so maybe they figured out how to do it(retina) and also offer a slightly faster edge. Would you buy a new iMac that was only slightly faster than previous version but had a retina display?
The last time I looked at the iMacs in my local store I thought the picture quality was absolutely amazing. So retina or not, I will be happy.. if there is than good it will be a nice bonus. Personally I would just like to see some USB3.0 integrated, and HDMI (as in the newly released rMBP) also the upgraded Ivy Bridge, GPU's, and standard SSD. If they do a complete overhaul I would like to see them keep the superdrive(others have stated they would like it gone).
Personally I like the current overall appeal, and looks of the iMac, I just want the latest internals really. But then again, I have not been staring at a 2011 iMac since it was first released ready for change so my opinion on that is biased based on I haven't had the luxury of having one.