This has been an interesting thread. Let me summarize what I think I've read.
1) If an iMac is the only computer you will consider, then then there are no less expensive, comparable computers (duh!). Maybe its because of its appearance, or because of its brand new 27" very high resolution screen to which others are just getting access. Using that logic, a $10,000 Intel 80286 purple and brown striped computer is the cheapest "comparable" computer for anyone that must have a purple and brown striped computer (because...I hope....that no one else sells purple and brown striped computers

).
2) If you are unfamiliar with an operating system, it is likely to seem unintuitive to you. "I ran Windows once in 1994 and it was terrible." It happens in both directions. I have been using OSX since Panther. The Finder was bad then, and it is still bad. You learn to use it as efficiently as possible, you install add-ins and alternatives, and wish for changes in the next OS update. Same for the Windows Control Panel. The Windows registry can be problematic, and so can digging thru the system trying to delete OSX plist files scattered around the system in an attempt to get some software to run properly. I used to feel that Apple was making more of an effort to improve things. Now I am feeling that Steve has shifted more of his resources into iPods, iPhones, and iPads, than he is to his computers, and Microsoft is the one making more effort to improve its OS (and you can argue that it had further to go). It's Steve Jobs' decision to make, and it could very well be the best one for Apple's bottom line, but it isn't be best for me as an OSX user.
3) Total cost of ownership involves more factors than just the initial purchase price, and it is there that the iMac shines. Someone should tell that to the 90% of the market that runs Windows, and to business users that I understand comprise an even greater percentage of Windows users. And if I had to bet, my bet would be that those business users have made more, and more detailed studies of the total cost of ownership than anyone has on this forum. And they have overwhelmingly chosen Windows. I cannot believe that they are all stupid...or that they have some inherent bias for Microsoft...ultimately their employment depends upon keeping their employer as productive and competitive as possible...whether its using Macs or PCs. And they have chosen PC's. So, either the PCs have a lower TCO, or they must be way, way more productive!
4) Windows users struggle with viruses and Mac users do not. This one worries me. I have always run antivirus on my Windows computers and haven't experienced any problems where I had to deal with a virus infection. I don't run anti-virus on my Macs, and also have never had a virus infection that I am aware of. Yet you know that there has to be vulnerabilities in OSX. Heck, like any OS, or any large piece of software, there have definitely been bugs of other types in OSX. And bugs in other Apple software. So there has to be security holes in OSX as well. I worry that one day someone is going to decide that even though OSX only has a ca. 4% market share, it will be worth their while to go after OSX. And when they do, it will not be pretty for us running without virus protection. Recall that Apple added rudimentary antivirus software to Snow Leopard....think Apple knows something?
5) iMacs are high quality, and PCs are garbage. Someone must have forgotten to tell that to all the people with yellow 27" iMac screens. Computers are complex devices. All brands break. Macs possibly at a lower rate than PCs, but look here and at the Apple.com forums. Just like on the HP and Dell forums. All brands break. If you buy a Mac, buy Applecare and you're protected until the computer is probably no longer useful. If you buy a PC...maybe extend the warranty...or maybe when it breaks in a couple of years just replace the part because you can get one inexpensively. Or, since it cost about half of the Mac, use the money you saved to buy a brand new, much faster PC.
6) Apple has great customer service. If you live near an Apple store, that is one that I agree with. As long as you pay the Applecare premium. If you don't have a store nearby, or if you're past your initial 90 days and don't have Applecare, then maybe not so much of a difference in customer service.
7) Finally, one question that was only touched upon. What have people done with their perfectly functional iMac displays when their logic boards eventually fail, or their processor, or their graphics card, or any of the parts that cost more to replace than buying a new computer? As one poster mentioned, I typically use my displays over 2 or 3 generations of computers. All in ones just seem so wasteful in this regard.
So, if you must have an iMac, then it is the least expensive computer for you. If you have some flexibility in your requirements (like you are only willing to run OSX, and you can accept a tower and a 27% Full HD display, or a smaller very high resolution display), then you can probably get a PC that meets your needs for about half the initial cost of an iMac.
In an earlier thread relating to the Mac Mini, I costed out an HP 2.5 GHz Core 2 Quad slim PC for the OP with 6 gigs of RAM, current vintage graphics card with dedicated video memory, 640 gig 7200 rpm hard drive, gigabit ethernet, wireless N, dual layer tray loading DVD with lightscribe, an empty PCIe slot for expansion, wireless keyboard and mouse for about $300 less than the cost of the 2.53 GHz mini. I saw a 27" 1920x1080 Samsung monitor at Costco yesterday for $350. So for less than half the price of the 27" iMac, one gets a computer that is competitive in speed, more RAM, a much better graphics card, a smaller hard drive, and an equal sized, lower resolution display.
Upgrade to an i7 for about $400, and to a 1 gig hard drive for $40, and for 25% less than the iMac you would get a computer that will blow the doors off of the Mac and have a very good, but still lower resolution monitor. Wait a couple of months, and the iMac's display will be available from 3 PC suppliers for another $150 over the cost of the lower res display, and the comparison will be more in favor of the PC.