With the release of the latest iMac, there are several key changes that change the iMac forever.
Firstly, the glass and LCD screen is now glued onto the alumiumium chassis. Which effectively renders the iMac non-serviceable and non-upgradable. In my experience as a Macintosh technician, the largest failure point of the iMac is the hard disk drive. The most often upgraded part is the memory.
Secondly, the hard drive in the 21.5″ model has been changed from 7200rpm to 5400rpm drives. There is not SSD option, save for a fusion drive option.
The design philosophy of the new iMac is the same as for the iPhone and iPad, pick which configuration that you want at the beginning of the products’ life and it will stay that way until the end of product life, no upgrade is possible.
Having said all of this, I do like the the design of the new 2012 iMacs and will be buying one for myself, however, I will be bypassing the internal drive and booting off an external thunderbolt drive instead, that way when the drive does fail, I can extract my data without permanently and irrevocably dismantling my iMac.