If I understand you correctly, you would want all distributions to switch to one UI? Or do mean the developers of the various UIs should all switch to work on just one?
Just like how everyone is using Linux as the kernel, so should, in my view, everybody get together and work on making a UI with a consistent look and feel, get that to a stable version, and then branch out by adding features / making modifications like transparent-like UI, old-style UI, whatever UI.
I don't see either option happening. The former would further diminish the differences between distros and the latter just makes no sense. People have different ideas and desires as to what they want from the UI.
If you use a complete DE (such as Gnome or KDE) things should feel a lot more cohesive.
Don't have much experience with diving deep into the architecture. But what would that common system be, the window manager? Because it certainly feels like there are a bunch of window managers, GUIs, visual styles, themes, and that they don't connect together to feel like one cohesive thing.
As for themes, they can feel less cohesive since the window decoration can be changed independently of the UI widgets. Furthermore applications can be developed with different widget toolkits (GTK2, GTK3, Qt, etc). Unfortunately there is not much you can do other than choose your applications carefully.
Well Linux distributions by their nature are much more modular and customizable than Windows or OSX. They let you switch out almost any individual component/package for another. In a way, distros are nothing more than a particular combination of these packages.
In any case, I find Windows and OSX to be more consistent. Linux distributions on the other hand feel like their parts have been hacked together to work, with the sense that if you touch them, the whole thing will fall apart. Maybe I am wrong, maybe the code is actually a lot better than OSX or Windows and works correctly, but it certainly doesn't feel like that. That is what I have from my limited use.