This question is for all you longtime Mac users who rose with me from the ranks of System 7 and before: In the switch to Mac OS X, what's the worst change, overall, that Apple made to the interface? What brilliantly simple rule did they throw away, thereby plunging us all, ever after, into a sheer hell of confusion and error?
If you said, "They allowed windows from different applications to become interwoven," that's a very good answer (and something I've complained of, for sure), but not quite the one I was thinking of. No, I was thinking of the introduction of clickthrough.
Back in the old days, when you clicked on a window that wasn't the frontmost window, it came to the front and that was all. (And, if that window belonged to a non-frontmost application, all of that application's windows came to the front right behind it; but, as I've just said, that's not what I'm concerned with here.) Today, on the other hand, when you click on a window that isn't the frontmost window, that window comes to the front, and there is likely to be a second effect: If the point you click on happens to lie within any sort of clickable interface element, that interface element responds to the click.
Thus, in clicking to switch windows, you can also accidentally trigger some other change, such as jumping from one Finder folder to another (because you happened to click in the Finder sidebar), or from one Web page to another (because you happened to click the Back or Forward button in Safari's toolbar). The effects are particularly insidious if they are not immediately noticeable: I believe, for example, that clickthrough is responsible for many mysterious mess-ups in my System Preferences.
Well, Leopard users, now is the time to cheer. Like Superman swooping down out of the sky to save the day, here comes Klicko, brainchild of Rainer Brockerhoff (author of Quay and other great utilities mentioned in my "Quay Sticks It to Stacks", 2007-11-27). Klicko prevents clickthrough. To put it another way, it restores the pre-Mac OS X behavior: when you click on a non-frontmost window, that window comes to the front and that's all.
I don't know how Klicko does its magic, but I believe it is taking advantage of Accessibility (explained in my "Scripting the Unscriptable in Mac OS X", 2003-03-10), because I turned Accessibility off (by unchecking "Enable access for assistive devices" in the Universal Access preference pane) and Klicko stopped working. I didn't explore any further; Rainer's site says that Klicko "doesn't hack the system, other applications, inject code or do anything magic," and that's good enough for me. What's important is that it works - and if it misbehaves for some particular application, you can try modifier-clicking on a window (which tells Klicko not to operate), or even exclude that application in Klicko's preferences.
Klicko is freeware; voluntary donations are accepted. It's a tiny 134K download, is a universal binary, and requires Mac OS X 10.5 Leopard. For more information, go to Rainer's Web site.