As witnessed with Apple's 15-inch Retina-equipped MacBook Pro, it looks like the company is slowly phasing out the ability to upgrade its products with faster and better internal hardware. More evidence of this claim can be seen by peeking into the new 21.5 iMac, where it seems nothing can be easily upgraded -- not even the RAM.
The new iMac is a beautiful machine to say the least. The 5-millimeter-thick edges make the iMac look impossibly thin, but the illusion is that most of the bulk has been moved toward the center, which no doubt took a lot of design work and rearranging of components. However, Softpedia is reporting that the new design comes with a cost, and that's zero user upgrades.
Apple chose to laminate the glass face of the LCD panel in order to make the machine thinner. However, this eliminated the ability to pop off a few screws and add more RAM. Plus, the RAM is soldered to the logic board anyway. Of course, the iMac's 8GB of RAM that comes with the base model is plenty, but if you'd want to add more down the road, you won't be able to.
However, the 27-inch iMac is still upgradeable, surprisingly. So, if you're wanting the ability to upgrade your iMac in the future, the 27-inch model is your only option, or you can buy the 21.5-inch model with 16GB of RAM to make sure that you never have to upgrade it. The entry-level 21.5-inch costs $1,299.