If you want everything to be exactly the same between all the windows, then just use KDE apps.
Lots of people do, I don't though.
As far as gimps UI goes, its more of a matter of personal taste. As I remember it learning how to use Photoshop it isn't the slickest UI in existance, but people treat it like it's its a god-like application, untouchable by mortal hands.
Personally I find it amazing how many people can afford a 300-400 dollar program just for occastional hobbyist stuff on their computer. You'd think that the price of a program would be a barrier to it's acceptance. But what do I know?
Linux != Windows.
Linux makes a realy crappy Windows. It's not a replacement desktop for Windows, it's not a free Windows and it never will be.
If you hold it to the same standards as what was designed into Windows it's going to fail miserably. Because it's not what it was designed to be.
The people who designed and created Linux, BSD and other free software hated being in Windows, and other commercial OSes. They disliked the statis quo so much that they decided the only solution was to sit down and write a entire OS from pretty much scratch.
Windows is dominated by large monolithic programs. Linux is dominated by a numerious small tools and utilities to can string together to be more powerfull then the monolithic programs.
Windows is designed around it's GUI. Everything is tied together with the GUI and the OS is unoperable without it. Linux the GUI is optional.
Windows is designed to isolate you from your computer. Many times it is even illegal to go around and modify the OS in fundamental ways, which is normally impossible anywyas because it's kept a big secret on how the OS functions anyways. Linux is designed to let you delve thru the layers of abstraction and fundamentally modify the behavior of the OS, it encoreges openness, documents even the most minute operations of the software and wants it's users to go in and screw around with stuff. Plus the code is freely aviable.
Windows assumes that programmers and developers are professional, and most of the OS is designed to deal with the lowest common dominator among it's customers. Linux it is a development platform first and a desktop second. The actual most effective way of interacting with it is thru the command line, the language is BASH and each line you type out is as part of a computer language, and bash itself is used to program and design the OS. By the simple act of using your computer, you are brought a deeper understanding of how it works. Check out the runlevel scripts for the most blatent example. Learn howto use the bash shell, then learn the basics of bash scripting and then you have automaticly obtained the mental tools needed to understand and modify the OS and develop it to your specific needs.
Another example of this is how Windows developement tools are add ons, and monolythic tools themselves. With Linux you have stuff like "vi", almost all desktop installations will include the GCC compiler and tools needed to make and build applications. In fact many Linux users have the intense need to build ALL applications they use from scratch. Just to get the extra yard or so out of them. Whatever.
Windows it's a single unified interface. Linux it is a fractured and highly customizable setup. For example Gnome uses Metacity by default, which is all nice and dandy, but it is intentionally restrictive. But it's still worlds above the Explorer shell in terms of customizability. But then the standards for the desktop is that you can set it up to use OpenBOX + Gnome, or Enlightenment, or Icewm, or Window Maker, or Wiamea, or FVWM, or Afterstep, or dozens of others that are compliant with a KDE or Gnome enviroement. And on top of that you have hundreds of other WM to select from.
So that makes for a fractured UI, but it also makes it extremely flexible.
If you hold Linux to Window's standards, then it sucks. But if you hold Window's to Linux standards, then guess what? Windows sucks too.
In fact all OSes suck, but what OSes suck in a way that you can find palatable? Some people will gravitate towards Windows, other Linux. The only sucky part is that MS designs it's OS in such a way that it intentionally makes it difficult to play with others. (work with other enviroments in a meaningfull ways)