With established norms stabilizing on the web, UI creativity there can and should decrease. However that said, there is always the possibility for optimizing towards narrower and more specific problems, but this must be done sparingly since new interactive paradigms introduct a cognitive load on the user which may not be acceptable for the intended audience.
And the real kicker is that most of these aren't done well.
Aside from having a very limited expressive range, flat design has all kinds of usability problems that plenty of people have called out from the start. The fact that both Microsoft and Apple have chosen to adopt flat design does not change that unfortunate reality.
I cringe a little every time I see a web site using Proxima Nova, a font which renders horribly at many sizes and in many browsers, whatever all the renting-you-fonts-for-money services will tell you about how optimised their fonts are. I wince even more when I have to use a site with trendy thin fonts that are so hard to read under many conditions that I have to start hacking the CSS around to make it more comfortable. At least there is a small silver lining here: Helvetica is less common in font stacks now, so everyone on Windows who has printer fonts installed can breathe a sigh of relief.
Carousels have problems that are well known to anyone who studies usability, yet people persist in using them, presumably because they care more about looking cool in a demo than having a site that is actually effective. Or they just don't know any better, because they don't know anything about usability, and anyway they feel compelled to use at least 98% of what's in their template on every project. After all, the only acceptable alternative is to use a hero image that automatically scales to full page width, thus reducing modern, good quality, high resolution monitors to displaying heavily pixellated versions of clip art that is probably irrelevant anyway, and obviously no designer would do something silly like that.
It's not the consolidation and uniformity that bothers me. It's consistently doing things that just aren't very good that I find frustrating.
Example: Windows 95, 98, ME, and XP all had a panel at the bottom with a menu ('start') button on the left. That became normal for a computer. Anything different (Canonical's Unity, Gnome's Gnome-Shell) is seen as 'problematic'.
Reference: Zen and the Art of Motorcycle Maintenance, the bit about the early history of motorbikes and the surprising range of arrangements.
The approaches offered by newer systems like Unity, GNOME Shell, and Windows 8 all fall well, well below these levels set by the earlier approaches, however.
They're seen as "problematic", to use your terminology, because they are inherently inferior to earlier approaches in many important ways. They leave users worse off. So of course they will be seen in a negative light.
I'm confused how that makes Unity "well below these levels" since their meta-key function is effectively a more robust search and their taskbar is a dock, that is just on the side of your screen.
Or Gnome, where their meta functionality is, again, search. And their dock just requires you to hit meta to access, as if its always hidden. And you get window presentation that way.
The only significant loss - and I would agree it is significant, maybe its one of the reasons I use KDE - is the lack of a categorized tree graph of all installed programs, for when you don't know what you want, you don't have it pinned, and need a way to narrow your options down. That doesn't happen easily in Gnome or Unity (you can present applications by group by searching the group) but a novice user wouldn't know that.