Hmm just my observations so far from my iMac:
Not possible in the iMac due to the various obvious constraints, but overclockable CPU would be good, the i7 2600k has massive OC potential for example, you can easily run it at 4.4 Ghz stable with zero messing with voltages needed, just change the multiplier and your done. Temps are even very good with the stock fan and heatsink! When paying for such a top processor you really want to be able to unlock all that power. Running at stock almost seems slow when you think you could easily at the very least unlock another 1,000 MGhz.
Mobile GPU also sucks a bit considering how much the machine costs and the size of that screen, no way you could put anything bigger in an IMac though, it would fry. The Hard drives in the iMac already run about 20c hotter than in a PC. Having seen the full version of the 6970 round a friends house, i have to say its quiet, couldn't hear it and never heard it go to 100% fan and it always runs at a silent ~33% even during most demanding situations with good temps.
Apart from that, probably more ports round the back would be nice, maybe a Blu-ray drive, (sorry Jobbs but i would like to watch the movies i already own without buying them all again off of you again buddy) the super drive sucks giant balls, and most of all, maybe a screen without any yellow ;-)