That's pretty ironic considering that if you take a look at XP, WMP11, MSOffice and Visual Studio you'll notice that not a single one of them looks like the others and that's just stuff from MS. Some of them even use custom open/save dialogs for no apparent reason. It's been my opinion for a few years now that Gnome or KDE are a lot more consistent than anything on Windows.
Exactly.
If you stick strictly to either Gnome or KDE you get a very consistant user space. Very nice. Much better then Windows or OS X if you want to get into it. Apple constantly breaks their own HIG for applications they ship by default with OS X.
Sure it's important, but it's not going to make or break anything. Typically apps actually require specific looks-n-feel for them to be good at what they do sometimes. If there is no good reason for them to be different (windows media player for instance) then they shouldn't be.
Apparently as it turns out the look-n-feel of a setup is very secondary compared to things like consistant configurations.
For example say your using KDE and you set Konqueror for the default web browser, Konqueror for the default file manager, and Kmail for your default email.
Now you go and install a propriatory package.. what does it do? Pull up Mozilla for browser and email.
And then you and use a Ubuntu-Gnome application and then what does it use? Firefox for browser, Thunderbird for email, and nautilus for file manager.
So _that_ is what is inconsistant about the Linux desktop. And this is one of the problems they are dealing with with Freedesktop.org. The look and the feel is not that important.. IF it's not important for OS X or Windows to be successfull then why would it be important for Linux?
A example of one of the ways they are dealing with this is the ".desktop" file specification.
If you are running a modern Linux distribution you can go:
locate ".desktop"
and it will spew forth a great deal of text files.
In each file there is specifications for how to launch applications.
Then when you go through nautilus, right click on a file and select a default action then you have this file created:
~/.local/share/applications/defaults.list
In there you have a bunch of mime types and then the *.desktop file to use to launch them.
Then also in that directory you can create your own .desktop files to create very custom launchers and then use them by default in any application that supports that .desktop specification.
That's _one_ of the things that people are doing to make Linux desktop more consistant and easier to use. It works out pretty well realy and it's easy to edit and fix if you have a problem.