As GBS (or was it Sting?) said, "we learn from history that we learn nothing from history".
When I first read about Microsoft's upcoming Vista and its new window manager, I was surprised to see mention of semi-transparent title bars. Surprised because earlier versions of Mac OS had this feature – one which seems like a reasonable idea until you try it – but newer versions of Mac OS have removed it, first partially (by making the title bar less transparent), and now completely (by making the title bar fully opaque again).
When Apple last year hubristically suggested that Redmond should "start their photocopiers", I doubt they expected Microsoft to still be catching up with ideas that have since been rejected.
Anyway, this misfeature seems to have survived into current Vista betas, and at least one reviewer hates it, and hates it for one of the reasons it didn't work on Mac OS. (There's irony in the linked-to author's anti-Apple jibe suggesting that "Apple can add stained glass windows to the next version of Mac OS X in response"; one can only assume that he, like Microsoft's developers, doesn't know that Apple already implemented and removed transparent title bars.)
If they're interested, the other reason why you realize that semi-transparent title bars suck when you comes to use them is that not only do they make it more difficult to recognize which window has the focus, they also make it more difficult to read the title bars because of the interference from everything beneath. This might be an even worse problem on Windows, where users tend to have all their windows maximized (and hence all their title bar text overlapping), compared to Mac OS users whose desktops tend to be far more random, reducing the chances of direct collision.
The cynic in me wonders if Microsoft doesn't see transparent title bars as a deliberate insurance policy to give users some reason to upgrade to whatever comes after Vista.
From the screenshots I've seen of Vista, Microsoft seem to have learned one important lesson from Apple, and it's a lesson Ubuntu need to learn. In Vista, the system tray (the bottom-right corner of the screen that fills up with icons for all the useless crapware that's installed on a Windows machine because some idiot developers seem to think that all users need quick access to such vital everyday features as screen rotation and virus scanning settings and mouse acceleration configuration and any number of other things that any normal human sets exactly once each time they install an OS) seems to have switched to silhouette icons rather than the current unintelligible low-res low-color (but gaudy) splodges we're used to from earlier versions of Windows.
Doubtless it will take years for the crapware vendors to follow Microsoft's lead – I'm pretty sure Microsoft explicitly says that most of their junk shouldn't sully the system tray at all – but it's a step in the right direction.
Ubuntu, though, needs to look at Mac OS' default "user" icon (a silhouette that's sufficiently abstract to be a generic carbon unit) and think hard about whether their default user icon – a black guy with a beard wearing a suit, in front of an old white woman in a baggy green sweater – is such a great idea. Those people are very much "not me" and very much "not users of my computer". Worse, there's a variant that's just the guy, that gdm(1) happily shows next to female usernames.
(Aren't there cultures that find beards and other facial hair offensive? I know I find the imputation that I might wear a beard pretty offensive.)
You can see the ugly beard icon in the "log out" dialog of Ubuntu 6.06 beta, by "Switch User". The suggested "tango" replacement is somewhat better, but still looks far too much like real people by giving them skin colors. The icon on the user-switching menu in A Look at GNOME 2.14 is infinitely preferable, and as asbtract as the little runner on the emergency exit signs in cinemas.
But what can you expect from a desktop environment whose default background is a big brown lake of shit? Unlike the beard icon, which I'm currently suffering in Debian, the "shit brown" leitmotif is Ubuntu's fault rather than GNOME's, which appears to be 100% skidmark-free. As an interesting experiment, you can check out user-submitted GNOME 2.14 screenshots and count how many people choose to have pleasing green or blue backgrounds compared to how many feature "oh no, I've crapped my pants" brown.
Yeah, I know: if I were doing work I wouldn't be able to see the desktop...