Apple's new Retina-based iMac is a thin, petite, great-looking all-in-one, but it does have one, huge flaw: it has no ability to upgrade the RAM after the purchase. Softpedia notes that Apple have removed the optical drive and laminated the glass face to the LCD panel.
This is great, it makes it thin, with 5mm-thick edges, but now the RAM can't be upgraded once you've purchased it. Normally there'd be just a few screws to undo, slap the new RAM in and away you go. The stock 21.5-inch iMac sports 8GB of RAM, but there are power users who would like the option of moving up to 16GB, maybe not one day-one, but down the track.
There are two options to get out of this, buy a 21.5-inch iMac with 16GB RAM at the time of purchase, or ride right up to the 27-inch iMac that does have user-serviceable RAM.