No! Wait! Come back!
I agree with the AppleXnet article on the whole, but I do have some rebuttal to it.
Apple sanctions the use of "extensions", which modify the use of the operating system in unintended ways that often lead to conflicts with other applications and extensions, one of the worst usability problems of Mac OS 9. Troubleshooting extensions is one of the worst things to have to diagnose.
Trivia: Extensions were originally unsupported. They were only for Apple to use to patch the OS without requiring a full OS update. But third-party developers figured out how to do it. Apple made it supported so that they could set forth certain ground rules to help ameliorate the problem of extension conflicts.
For disabled users, there is no way in Mac OS 9 to click any menu item or drop down any menu without using the mouse or buying additional software. This makes using the operating system very difficult out of the box for many disabled Mac users.
OS 9's Easy Access control panel offered a feature called Mouse Keys. You use the numeric keypad to move the cursor, except for the 5 and 0 keys, which click the button. OS X has it too, since Jaguar.
Exercise for the reader: Figure out how Mouse Keys changed between OS 9 and OS X. (Yes, there is at least one change.)
While admittedly minor, some applications' menus are white colored, from the System 7 days, while other ones are gray colored, the new color scheme adopted by Mac OS 8 and later.
The applications with white menus used custom MDEFs (menu definitions), most commonly an old version of Mercutio. These custom MDEFs were usually employed to allow for exotic key combinations that Apple's MDEF didn't allow until 8.5.
Mac OS 8.5 and later, and OS X, allow any key combo.
A remnant from earlier systems from Mac OS 9 is the desk accessory, which violates many of the regular human interface guidelines (even though it's usually just a small application). For example, the Calculator has a black menu bar…
First, you mean title bar.
Second, its appearance is because it's a desk accessory. DAs didn't always behave like applications; before System 7, they floated on top of the active application. The reason is that DAs were the very first way in which you could do things that weren't actually part of the application you were in, invented because this was before MultiFinder allowed multiple applications to run at the same time.
Third (and irrelevantly to UI issues), DAs are not applications. They are device drivers. (Yes, really. That's how they were loaded when you could only have one application running at a time.)
Sounds a little like input managers, actually, now that I think about it…
The Apple menu and application menu under Mac OS 9 are curiously not located at the top-left and top-right corners of the screen, respectively. For some reason, Apple decided to put about 10 pixels between each menu and the side of the screen, violating Fitt's Law in the process.
But you could click on them anyway.
Immediately after startup after a crash, the Finder often places "rescued items" in the Trash. This is wrong for multiple reasons: …
Ostensibly, the application saved your unsaved work to a swap file that it would have cleaned up when you saved or closed the document, and you could recover this data from the Rescued Items folder after the restart. Unfortunately, even when there was such a swap file, I was never able to open one and recover anything.
Good idea, but it didn't work out.
OS X does have the same feature, but most applications do not use the Temporary Items folder anymore. I think developers figure that Macs have too much RAM for it to be worth anything anymore.
Pop-up windows violate most of the normal expectations that a user has when interacting with them.
Ah, but they were so handy. :)
Clearly-defined resize widgets.
It has not one but TWO resize widgets.
For no reason at all, the icons of desktop printers cannot be changed.
The reason is that the icon may change to reflect information. Most of these can be accomplished by badging, but what do you do for the black stroke that means “default printer”?
It's puzzling why an application called "Finder" doesn't contain a feature that does what it's name implies, instead relegating that to Sherlock. (Earlier versions of Mac OS X also carried over this problem, but it has since been rectified in Jaguar and improved in Panther.)
And ruined in Tiger. :(
Good thing there's NotLight.
Many diehard Mac OS 9 users that I've met seem to adopt an attitude of thinking that Mac OS 9 is perfect and should be Apple's current operating system, when Mac OS 9 was full of more usability problems than the current version of Mac OS X.
I was there. The first time I used OS X (10.1.3), I hated it for all the ways that it was different from OS 9. Eventually I went back. But I came to miss all the things that were good about OS X, and also wanted to start writing Mac applications (an expensive proposition on OS 9, except with MPW, the compiler in which was creaky; free on OS X). So, after getting a newer G3 (333 MHz; my previous one was 267 MHz), I installed OS X 10.1.5, and never looked back.