If the optical drive is dropped, I am not buying it.
Also, I prefer aluminium to glass, as it is better for heat distribution and is also more durable and recyclable. The Apple logo at the bottom also doubles as a infra red receiver and also polishes off the design nicely. I like the curved back and I think the thicker flat panels of the iPhone 4 and 4S are not as good.
They say the iPhone 5 will have a curvier back you know.
HDMI, USB 3.0, Dual Thunderbolt and spec bumps would be great.
I hope 512GB SSD comes as standard, but it's not likely. A retina iMac would be cool, and that's probably why Apple didn't update the iMac this July, since they are developing the Retina display, and any updates would turn out a disappointment like the Mac Pro.
A bigger cooling port would just be stupid as the current one is fine, and nobody can get their finger caught in it.
I do not want the iMac to be thicker than it is already.
8GB standard RAM would be awesome, as well as a 1TB drive on the base model.
I hope they continue to use AMD graphics as well, because I don't like NVIDIA ones.
The 'chin' is also the part of the iMac that balances off the whole design, and a centred display would just look out of place.