Ah, the chin! What's the big deal about the chin? I have one, and it seems even larger because I wear a beard.
OK, I was glad to see the chin disappear from iPhone, but the trade-off there was more screen real estate. I doubt that Apple would fill that space on iMac in order to deliver a 4:3 aspect ratio. I'd guess that if the chin went bye-bye there would be nothing but open air to replace it.
Practically speaking? I guess the iMac's chin is driven by thinness. Would there be room behind the display panel for the speaker assemblies, fans, and main logic board? Yes, if the unit was twice as thick as it is. The available photos show, however, that those components are in the chin. You might say, "Chin makes thin."
So queue the "Apple is obsessed with thinness" comments. It's certainly not essential that iMac be that thin - my 27-inch has that deep bulge in the back, and after living with that for many years... the new, totally flat back looks sorta svelte. It also makes it look like an oversized iPad Pro, which isn't necessarily a bad thing.
One thing about the chin is that it helps distinguish iMac from a simple flat screen display. If you have to contemplate how the thing will look when on display in a store, that's not an inconsequential consideration.
Thin was likely the primary motivation for the MagSafe power connector - there's not enough depth to accommodate the familiar IEC mains power receptacle. A lesser manufacturer might have just equipped the thing with a non-removable power cable, but that also necessitates country/region-specific models with the appropriate power plug on the other end. It's still more efficient to produce one, world-wide configuration and drop an appropriately-plugged power cable or power adapter into the box. And as an added bonus, folks love MagSafe and that satisfying click when it leaps from your fingers and fastens itself to the machine. Quick-disconnect isn't as important in a desktop as a laptop, but it'll still prevent some unfortunate accidents. And wall warts/external power supplies do reduce internal cooling requirements and bulk (there's that thin-thing again) even though they do clutter up the externals.
As to the side-mounted headphone jack? Not too many reasons for complaint if you actually use it for headphones - it's a much better spot than the back. As others have noted, thin may have been the driving force of this move, too, due to the depth needed for that jack. However, unless you're an audio pro who's connecting the Mac to an analog audio interface box (instead of a digital I/O???), side-mounted seems far superior to rear-mounted.
I've owned every iMac form factor other than the original flat panel with the hemispheric base - the CRT-equipped Lime (1999), the thick-sided white plastic 20-inch Late 2006 (Intel Core Duo), thick-sided silver aluminum/black plastic-backed 20-inch Early 2009, the thin-edged, all-aluminum silver 27-inch Late 2013 (and several others in that factor from later years). I'd be delighted to have a 30-inch with the same design as the new 24-inch. Maybe it'll be the same thickness, maybe a bit thicker to maintain proportion. I guess we'll just have to wait and see.
Yeah, looks are only skin deep (and this thing is so thin it's barely more than skin-deep), but if nobody cared about looks nobody would be complaining about the chin, either.