I don't edit movies, but the INCLUDED iLife software is incredible...you can do anything with it...music,photos,videos,movies, etc. It's super easy to use yet incredibly powerful at the same time.
The best thing about the iMac in my opinion is the design. No other computer is designed so beautifully and uniquely. The iMac is pretty expensive, but it's not a cheap computer you're getting.
Mac OS X is nice because I rarely ever get the "program not responding" message that is so common in windows. And if an application stops responding and you force quit it, it doesn't stop all your other stuff you have running - just that one application. So you're not as affected by system crashes.
Another reason that should convince you to buy a mac is that people know a LOT about them. If you buy for example a dell x593fh3jkf model (that's an exxageration...) youre less likely to find help from people with that specific model. If you come here, there's a whole forum just for iMacs..and all the people here are mac fanatics who truly love computers. You can get incredible help here, if you need it.
Another plus: plug something in, and it really does just work. I plugged in my printer when I got my iMac and thought that the USB ports were broken...! That's because it doesn't bother you or anything, you click print and it just prints - no configuring necessary. Another example - I have two digital cameras. One of htem is a few years old, and yet somehow it still just got "recognized" by iPhoto right away. Very slick. I will note, however, that my scanner was so old that it did require that I install software (designed for Mac OS 9, but still worked!)
I think overall it's the fact that Mac OS X offers the tighest integration between its applications. For example, in every application/window, you get 'spotlight' which is like windows search, but better because it pulls up results instantly as you type. And about integration - you plug in your camera, open up iPhoto, camera pops up, click "import" and done! It files them away nicely for you, too, so you don't end up having a big mess of jpegs all over the place.
Integrated wifi/bluetooth is nice, too, because you can get a bluetooth mk set and it takes away desk clutter.
Also, you don't need to worry about viruses/spyware. Although I really didn't mind updating my antivirus with windows, I realize now that I spend a lot more time just using my computer rather than maintaining it.
Integrated iSight camera is great - it works really great if you know someone with Aim or another mac. Video conferencing is painless.
Another thing: when you first get hte Mac, you don't need to install the "PC decrapifier" (google it...). No junk/bloatware.
Macs aren't what they used to be...there are no "compatibility problems" between common file types, and they're not "slower", or even in most cases, more expensive.
Although my iMac still is audible, my room is much more quiet with it in there. my old PC was super loud and noisy.
There are a lot of nice touches to OS X you may not realize right away. Fundamentally, it's a very easy OS to learn..it took me a few hours to understand most of how it works. But then you start realizing that there are many cool implementations...double click a word to hilight it, and theres an option for looking it up in the built in dictionary, or even to run a search with spotlight. It's things like that you only learn about after having the mac for a little while.
"Expose" is another feature that draws people to Macs. If you have a few windows open, you can push a key on the keyboard that you programmed (F1 for me), and it organizes all of your windows so that you can see all of them at once (i.e. no overlapping windows). I push F2, and it hides all the windows and brings me to my desktop. A small, but extremely useful feature.
Wait for WWDC to buy - you might get a nice software update with it (and maybe some more stuff). Also, look out for the free iPod/printer promo they do.
Oh yeah, it also makes that lovely startup chime