I used to build computers for myself when I was younger and I am very comfortable upgrading stuff. I managed to take apart my mini to change one of the HDDs to a SSD recently, but the fact that I would have to muck about with a screen on an iMac puts me off. It's like kryptonite to me.
I would love to see more powerful GPU options in the iMac, but how would they handle the extra TDP and moving that hot air out? Then again, I was reading the other day about some company that is now making fans that are "near silent" so where there is a will, there is way.
Also, there have been some interesting breakthroughs recently in the world of cooling, but I don't think they are ready for the mainstream, yet.
For example:
http://www.theregister.co.uk/2012/04/11/nanotube_heat_transfer/