|Well, mojoe has written about All-in-One crap, iMacs pt deux. I will give a good counterpoint on how it is a difficult trip it has been to assemble my own computer. I like my 'puter and I have learned a lot from building it, so I am not slamming this idea. I'm only reporting parts of my experiences and the hypothesis of an person using a run-of-the-mill PC.
The price of important parts
It's a pain to buy new and very important parts like the CPU and the motherboard. Although PC's have a variety of parts for one to use, the most important parts are still made by a privileged few. I can only buy old CPU's from Intel, and AMD stuff is not usable without a separate motherboard. Even worse, the Adaptec company is the only well-known company that sells SCSI adapters, but they charge an arm, leg, and a liver for me to use something to hook up with my CD-ROM drive and hard drives.
Oh crap, I forgot to mention how expensive it is to buy parts separately in my case. The sales tax goes up to my neck...
The dependencies of parts
I think the noder crowd would at least understand the concepts of different computer parts and how they're hooked up, but the regular computer user takes that for granted. Not everybody will know what does a PCI or AGP slot looks like. Not everybody will know that SDRAM DIMM modules replaced the SIMM modules of yesterdecade. If these users has never seen the innards of a computer, they wouldn't care. Apple has reduced the need for a user to learn about these dependencies by denying them the variety. It's a mixed blessing for Apple computers, and a mixed blessing for all IBM-compatible computers.
Okay, somebody can just call up Dell, Gateway, or some other computer company if in case they did some upgrade work with their respective comptuer. As long as the original modular parts are supported by the companies, users will not have to always be in the dark for upgrading computers. I have forgone the blanket of support from such a company because most of the computer building work is on my own to begin with.
Installing Linux is great; just install the bootable CD-ROM distribution to the computer's CD-ROM drive, and see what happens. However, for Windows users, they would have to find a store that sells the $200 (or higher) software for Windows and its more defective derivatives. Apple tied their hardware with their software. The software is included with the computers (excepting the obvious OS upgrades). Okay, you can use LinuxPPC, but that's another story.