The only revolutionary feature of an iMac would be retina display, it would be 4k on 21.5" and 5k on 27".
But then again I don't think it's going to happen anytime soon, not when high end desktop GPU out there barely able to play intensive games on 2560x1600, let alone 4K.
TBH though, there's no more revolutionary about desktop products, everything's about mobile devices these days and I'm afraid our desktop computing days are numbered. Yep, the infamous post PC era.
Computer OEMs start to acknowledge desktop computer as all-in-one. Yes iMac is still the king in this form factor, but my point is desktop is getting more and more dwarfed to the point it might be obsolete.
How many people own a quad GPU these days? Yep they're insanely fast but also drain your wallet and power bill just as fast.