Retina display.
New Ivy Bridge CPU's announced for late April release - There's a whole range of 4 core/8 thread options which are likely to be put into a new iMac.

Apart from the obivious CPU/GPU upgrade I would like to see inclusion of USB 3.0, HDMI port, SSD as standard and slave 1TB HDD instead of the Super Drive.
All seriousness why would you even need a retina display on something like a 27 inch screen? I currently use a 24 inch dell and from my viewing position the letters are clear and crisp. From 3 feet back what exactly would you gain with a retina screen?
It's not like a tablet where you are zooming in often.
USB 3.0 is standard on Ivy bridge so Apple can't really not include it (would be harder work to not have it). HDMI can be achieved atm via a thunderbolt adapter as can VGA/DVI so no need for a dedicated port. You can already get them with SSD+1TB HDD and the Super drive its just an optional extra as it will be in the 2012 iMac as Apple don't see SSD's as a necessity (prob because the ones they use are so slow anyway).
Oh and the 2011 iMacs have a matt screen, they just have a glossy sheet of glass in front of it, a lot of customers have simply removed it (held on by magnets) which looks a bit ugly but gives you a matt screen.

OMG... it's the ipad 3 thread all over again.
Why would you NOT want a higher resolution screen for crying out loud?
Zooming in has nothing to do with the resolution of the screen... that's to do with the source content.
Ignorant technophobe numpty...
Why would you NOT want a higher resolution screen for crying out loud?...