This is my first iMac so I can't compare it to the previous versions except the ones I saw at the Apple store when comparing the overall dimensions and thickness of this latest 2012 iMac.
PACKAGING - Includes 21.5" iMac, Charging cable, Wireless keyboard, and Magic Mouse. Like most Apple's products, the packaging is thoughtfully done. All tape pieces are easily removed with the individual tabs attached to them. After set up it has a clean look on my desk, there is no clutter due to the fact that the keyboard and mouse connect wirelessly. Opening the box to setting it up took maybe 5 minutes tops.
FORM FACTOR - This iMac is super thin from the sides and light enough for one-handed portability if required. Looking at it from the front, it seemed only marginally different from its predecessor but the difference was very noticeable from the side. The back is slightly curved at the center and I am guessing all the components are packed in this bulge at the center back. The front glass seems to sit more flush with the body compared to the earlier version which had a tiny little gap. Has a super sleek look to it overall.
SCREEN - Perhaps the most pleasing aspect of this iMac is its screen. Given the amount of real estate you get combined with the stunning LED-backlit display, it's a pleasure using the screen with its vibrant colors and low glare. Apple claims this model has 75 percent less glare than previous versions - there's no way for me to measure this but suffice it to say that even with the screen turned off, the screen is not really reflective. Once it's turned on, the reflections die down even further. Images are crisp and pictures we have taken with our DSLR camera seem further enhanced given the resolution of the screen.
PORTS and CONNECTIONS - Apple did away with the optical drive in this version, which means that to read/burn CDs or DVDs, we can either connect an external drive like Apple's Superdrive to the USB port, or feed off another PC/Mac (this has to be on the same network) that has a built-in optical drive. If you are considering getting an external drive, I personally would recommend the Samsung SE-208AB/TSBS External DVD-Writer - Black which performs the same function and is $40 cheaper than Apple's super drive. For the initial set up, I was able to wirelessly transfer all files from my old PC to the iMac using the "Migration Assistant" utility on the iMac - it took a minute to start the process and all files transferred themselves in about 6 hours over the Wi-Fi network. Didn't have the need for an external drive, though that would have been a faster process.
On the back, there is a SDXC memory card slot, couple of Thunderbolt ports, headphone jack, USB ports, and an ethernet port. It's good they are at the back since they avoid cable clutter from the desk, but connecting wires into them takes that small effort of getting up and plugging in the cable at the back. Not a major issue for me, the only port I will need to use is the USB port for my printer which doesn't support wireless connection.
HARDWARE - Tech specs are all specified above in the product details of the listing. That said, with a 1TB hard drive, 2.7Hz quad-core i5 processor, 8GB of DDR3 memory (split into two 4GB slots), this Mac basically has more than enough in terms of processing and storage capacity for my needs. Graphic designers and users that require super memory or processing prowess, they can perhaps judge for themselves if this will suffice.
ACCESSORIES - The iMac comes with a wireless keyboard and a magic mouse. The keyboard is a standard Apple keyboard, but I have found it to be slightly small for me personally. Takes some getting used to. Instead of using the Magic Mouse, I had considered getting Apple's Magic Trackpad because that is very similar to the MBPro trackpad that I have used extensively. However, having used the mouse for a few days now, I am sticking with it - just a personal preference. The Magic Mouse also lets you scroll down lengthy web pages without having to move to the side to grab the scroll bar. Just a swipe of the finger on the mouse moves the page down or up. It can also be configured to register right clicks. Somewhat similar in terms of functionality to the magic trackpad.
BOTTOMLINE - There are two versions of the 21.5" iMac, one with a 2.7Hz i5 processor and the other with a 2.9Hz i5 at an additional $200. I went with the 2.7Hz which is good enough for me personally. Given the sleek form factor, absolutely gorgeous display, pretty good quality built-in speakers, front facing camera, and more than sufficient technical specs for the ordinary user like me, this is a great buy. If portability is not an issue, this in my opinion is definitely the device of choice over the MB Pro.