I helped administrate several classrooms of OS X machines around the 10.2 release of Macs.. And I can tell you from personal experiance dealing with hundreds of mac machines that there is significant differences between 10.1, 10.2, and 10.3 versions of OS X.
Nowadays I dont' have much to do with OS X, but I don't expect any other from the rest of OS X's upgrades.
Each revision is nearly equal to the difference between W2k and Windows XP. Not only in UI improvements and new application features, but in api stuff and other low level things.
For instance you had the printing subsystem completely changed out. zeroconfig networking improvements, they changed over to a entire new computer architecture, they had several transitions were they took their Aqua interface which was originally software driven and moved to a completely opengl accelerated interface, and bunches of other stuff I can't remember because I haven't used it in so long.
The reason most people don't notice this stuff is because Apple are the kings of usability when it comes to commodity operating systems and they made sure that the user wasn't affected.
There are several reasons why Apple is able to keep changing and releasing new systems were Microsoft is relatively stagnant...
- Apple's focus is much tighter on their audiance. They concentrate on desktop systems for home users and small workgroups. Microsoft on the other hand has to take large businesses into account, as well has hundreds of computer vendors, individual users, workgroups, corporate domains, and all sorts of crazy stuff that very simply Apple doesn't give a crap about.
- Microsoft cares about it's third party developers. They work hard on backward compatability, for instance. One of Microsoft's greatest strengths is the fact that they offer a unified platform for application development.
They don't have to worry about dependancies or library versions, or not having this utility or that utility present... This is because if people are running Microsoft Windows they are either running Windows 2000 or Windows XP and that's all you have to care about. There are no real standards, no POSIX thing you have to worry about, no weird industry group made up of competeting corporations were you have to pay 5000 dollars to join in order to learn how your apps should be programmed (and which doesn't usually work anyways). With programming on Windows you program against windows and if it works then it works. That's all you have to worry about. Microsoft does a decent enough job of making sure that it works across multiple windows versions now and into the forseeable future.
This is probably the significant reason why Vista had a lot of it's longhorn features dropped and one of the major reasons why security for Windows is very difficult.
In comparision Apple just doesn't care. They use a slick interface to market their computers to a audiance then turns around and demands application developers just have to keep up with Apple. There is very little garentee that complex applications, besides the more popular items such as photoshop or quark, are going to work between versions.
(This is not to say that applications can't be expected to work across multiple versions of OS X, it's just that they can't be trusted. This is from personal experiance.)
Apple regularly takes improvements introduced by third party application developers and incorpoate the most important parts into their own system. They've put a lot of smaller programmers out of business and made lots of shareware and payware applications irrelevent with each new release they do.
This is very distructive, but it helps Apple sell new computers.
- Apple leverages open source software and has other people do a lot of the work were Apple then is able to concentrate on the portions of their OS that have the most impact for end users. Microsoft on the other hand is in charge of producing the entire OS.. As well as other OS and software stacks that are not core to Windows, but are nessicary infrastructure for Windows and Office users in the enterprise environment.
- Windows is more sophisticated then OS X and is on many orders of magnitudes more complex then OS X is. It's more difficult to work with, more difficult to do major upgrades.. Especially with application compatability and other support restrictions/promises that Microsoft works hard on keeping.
- Microsoft has a singificant problem of keeping windows compatable and functional across millions of different combinations of hardware. Some stuff that is very specialized and for target audiances. Apple only has a few types of computers to worry about.
And stuff like that.
There are definate reasons why Apple is able to release OS X upgrades more often then Microsoft is able.