Apple made major tradeoffs in order to dramatically thin down the new iMacs. The iMac joins the MacBook Air, Mac mini, and Retina MacBook Pro as computers that no longer have internal optical drives. If you need to use and burn CDs and DVDs, you need to attach an external optical drive, like Apple's $79 USB SuperDrive, an 8X DVD burner that runs on USB bus power.
User-accessible RAM slots are also gone on the 21.5-inch iMac. The standard-configuration iMac comes with 8GB of 1600MHz DDR3 memory, which should be plenty for most people. Last year's model had 4GB of slower 1333MHz DDR3 RAM. If you think that you may want more memory down the road, you should customized your order so that 16GB of memory is installed at the factory--a $200 option.
Adding RAM after your purchase will most likely require a visit to an authorized Mac repair shop. Getting into the old iMac was no easy task; you needed suction cups to help pull off the glass, which was held on by magnets. The new iMac is even more difficult; the glass is attached using an adhesive strip that must be cut to open the iMac, and that strip needs to be replaced in order to close the iMac.
The new iMac moves the handy SDXC card slot from the left edge to the rear, next to the USB and Thunderbolt ports. FireWire 800 ports are no longer available, and you'll have to use an adapter like Apple's $29 Thunderbolt to FireWire 800 adapter to connect FireWire devices. The new iMac has the same number of USB ports as the 2011 model, but the four available ports are now of the speedy USB 3 variety, instead of pokey USB 2.0. The USB 3 ports are backwards compatible with USB 2.0, should you have such peripherals. The new iMac still has a gigabit ethernet port and a headphone/audio-out port on the back, but an audio-in port is no longer provided. The new iMac gets a second Thunderbolt port, which is very convenient if you want to connect multiple Thunderbolt devices.
The new iMac also sports redesigned speakers. Setting the new and old iMacs side by side, the music playing through the new iMac sounded noticeably warmer and fuller than the previous iMac. The older model sounded a bit louder, but shrill in comparison.