UI consistency is sadly a pipe dream in the era of a dozen platforms which all want to "develop" and to "differentiate" themselves and applications which are ported back and forth as the OS fads change.
True, though it was more internal consistency I had in mind, not so much consistency between platforms. But your point is well taken. (I do find it interesting that in areas where the platforms have become more similar, it’s often been the Mac’s way that eventually took over. For example, how Windows now handles mouse scrolling and some text selection behaviors. Though examples of the opposite definitely exist, too — like keyboard shortcuts, which the Mac didn’t even have until Microsoft put them in Office for Mac in early 1985!!!)
And an elephant in the room is that even the individual platforms aren't exactly UI-consistent over sufficiently long timespans, while a lot of professional software dates back decades.
Ain’t that the truth!!

As a long-time Mac user (since 1992), one of the things I dislike about the Apple of today is its much less rigid adherence to its own UI guidelines, as well as the watering down of the guidelines themselves. So while I still think the Mac has the best desktop UI overall, it’s not quite as
predictable an interface as it once was. (Since that’s the whole point of UI consistency: total predictability of behavior.)
Welcome to the brave new world of UI diversity 
Well, I wouldn’t call it
new. We had a lot more UI creativity going on in the past. Pretty much every desktop UI in common use now is based on the original Mac’s UI fundamentals, which MS copied into Windows, and then Linux et al. copied from Windows. The only other branch of UI that exists in any way whatsoever is X-Windows from the classic UNIX workstations, but that’s practically irrelevant now.
By the way, you get used to it over time. The amount of UI tricks invented so far is finite, at some point they just stop surprising you anymore.
Nope. Some things I’ll never get used to. For example, I’ve been using Mac OS X full time since 2002 or so, and I
still instinctively press ⌘-N to create a new folder in the Finder (as it did in classic Mac OS), but instead get a new Finder window. I don’t think I’ll ever un-learn that muscle memory.
It's all the genuinely novel fads coming from professional UI designers that I find most disruptive 
Well, I think those come from designers who approach it from an artist’s POV, and not from the user’s. Gratuitous change does nothing but stroke the designer’s ego; IMHO, a gifted UI designer is focused on what works best for the user, and most of the time, that means “don’t be creative, do what users know and just reuse existing UI elements/behaviors”. That and look at what users actually try to do; if you see tons of users trying to do something one way instinctively, it’s worth considering making that a way to do it!

(Yes, I’ve worked as a UI designer. It bugged the hell out of me when good designs got shot down because they were “the way everyone does it”, or even just — I swear to god this is verbatim — “it’s not Swiss enough, [the usual, proven way] is too American”.

Like... a good UI is a good UI, period.)