Unless you've just come from under a rock, you've probably heard that Apple has upgraded its entire desktop line, including a
newer, faster, 24-inch iMac. If you've never seen a 24-inch iMac in person, let me try to put this into perspective-
Here's a Hummer with 24 inch spinner rims:
Here is a Girl's' Dual-Suspension Mountain Bike with 24-Inch wheels:

Here is a 24-inch channel catfish:

Here is the world's biggest hamburger. Yep- its 24 inches across!

Here is a 5-piece drumset with 24 inch bass drum:

In short- 24 inches is HUGE! (two feet, to be exact).
Who really needs a screen the size of two Subway sandwiches laid end to end? I use a 17" iMac on my desk at work and it works out just fine for day to day work-related tasks, which is about 50% email, 30% researching info on the web, and 20% Word/Excel. I rarely feel like I need more desktop real estate, but I rarely watch any video on it either.
At home, I use a 20" iMac. I often us it as my "tv" and watch fullscreen video, lounging on a sofa 12-feet away with no complaints. (why I have no TV is a totally different conversation!). The only time I pine for more desktop real estate is when I'm editing videos, websites, or photos, which is 75% of the time I'm in front of that computer.
My friend is a music teacher and semi-professional musician. He does a lot of composing and writing of sheet music which is how he justifies his monolithic, 24" iMac. Even next to his 32" LCD TV, the 24-inch iMac looks huge and slightly out of place. When he is using the computer like "normal" person (email, web-surfing, Facebooking, etc), about 50% of the 24-inches is wasted real estate.
However if you spend most of your computer time editing music or videos with programs like Logic or Final Cut, you really do need all the desktop real estate you can get (Final Cut Pro as shown below). Which is why the newer faster (same priced) 24 inch iMac is really a good thing, for the right person. But for the average emailing, web-surfing, Facebooking, YouTuber, 24-inches is a little over the top. About 4-inches, specifically.
(Above, a desktop computer using Final Cut Pro)

And now, the prediction! When the first 20-iMacs came in 2004, I predicted that the day Apple announces a 30" iMac,
it will have a TV tuner and/or Apple TV built-in. Yep, you heard it here first!!