I love how Apple has maintained uniformity in their UI across the past few iterations of OSX. It's just one less thing to worry about relearning, allowing me to get my work done easier.
Apple is simply pushier, dropping support for old versions almost immediately. They also release more often, and I would say that over the same span of time their change drastic-ness is similar (intel, cocoa, app store, etc). I wouldn't be surprised if 10.9 vs 10.5 is as drastic as Windows 8 to Vista, for instance.
They don't, though. Vista was, cosmetically, a big leap from XP, but in terms of actual UI it barely changed at all. Start button is still in the same place, maximise, minimise, etc. All still there.
I do really appreciate the little touches MS add, actually. Drag a window to the top of the screen to maximise, to the left to use 50% of the screen, shake the window to minimise all the other ones... I use that stuff every day, and miss it when I go home to OS X.
That being said, its also easy to get stuck in a rut and have that kill your project. Microsoft has perhaps more scars than many in this area. If you recall they did a 'tablet' version of Windows long before iOS, but they failed to 're-imagine' and instead they tried to 'shoehorn' ideas which didn't make sense (we'll assume you have a high precision pointing device like a stylus or mouse) and so those preconceptions prevented them from seeing what needed to be done.
Mostly I think the messaging about the flag and what not is PR to try to tell people "Hey, we're awake and we're trying to innovate, and we want to be better, and we're sorry for Vista." If their stock price is any indication they are having some success there.
Even for Windows 7 checkboxes, I've seen capacitive screen drivers/software that allow for some "fuzzing", where if you hit somewhere around the checkbox/button, it gets checked.
Raymond Chen posted some reasoning for this on his blog: http://blogs.msdn.com/b/oldnewthing/archive/2004/05/25/14125...
If they don't change anything visually then nobody thinks anything has changed under the hood.
The public seems to have gotten along very well with the Windows 7 UI over XP without much, if any re-learning.
Vista was different because it actually changed the security and driver model, breaking a lot of hardware and software. I haven't seen too many complaints about the actual UI.
By "everytime they iterate", you mean Windows 3.1 to Windows 95(very well received) and Windows 7 to Windows 8(desktop mode is exactly the same) which is what, twice in 20 years? Maybe thrice if you count the taskbar changes in Windows 7.
The search box on the start menu was itself a huge leap over XP. That was the single big thing I missed in XP.