You can update it by yourself (Although you'll void your warranty), unless Apple choose to solder everything onto a single PCB and you have to replace everything if one of them failAnd a computer I cannot upgrade.
For the same price as the iMac, I can get something that has double the power AND can be upgraded overtime meaning that I'll always get the latest parts. Once you get the iMac, your screwed into purchasing another $1299+ computer in a few years when you could have just spent $100 - $350 on a new graphics card / processor.
And don't say "just get it because it's an iMac." iMacs use the same damn parts as other computers, and probably cost a whole lot less for Apple to purchase due to bulk purchasing, meaning Apple makes hand over fist money by screwing their customers into purchasing non-upgradable computers.
They're doing the same with the Mac Pro. Apple doesn't want pros upgrading internal parts because they won't have to buy thunderbolt to do so. Thunderbolt is even more money into Apple's pocket. Why do you think there's six thunderbolt ports? That's an extra $234 JUST in thunderbolt cables.
Just take a look at this video:http://www.youtube.com/watch?v=_tNrfO-DqwA