I can think of several reasons for this, but there are some aesthetic problems in KDE applications which are fairly easy to deal with, and they do make a noticeable difference. So it is worth fixing them.
Something that was picked up in comments to various kde-look.org mockups for KDE 4 is that they look clean in part because they don't have many of the ugly frames and borders which litter KDE applications at present.
The majority of these frames and borders are not deliberately put there by application developers, they arise as a by-product of the way in which the user interfaces are constructed. For example, a tab widget is created, which is filled by a list box. Both the tab and the list box have borders by default, and so the on-screen effect is an unpleasant double-thick pinstripe border. KDE 3's default Plastik style makes this worse because it uses thickish 3D borders for quite a number of user interface elements. Gnome's aptly-titled Cleanlooks has thinner, less visually pronounced borders for many elements, which helps somewhat.
Removing these borders is, for the most part, easy to do.
Below are a couple of screenshots of Akregator. The first is the KDE 3.5.5 out-of-the-box screenshot. Both the list box containing the list of feeds and the article viewing area have borders around them. The second has both of these removed.
Akregator (KDE 3.5.5)
Akregator (patched to remove frames)
One persistant offender is Qt's tab widget. It has a border around it which I have not yet been able to turn off. This is the fellow responsible for the 3 or 4 pixels of dead space at the bottom of the Konqueror window, and also the reason why Konsole still has a border around the display widget if you hide the window decorations ( the associated bug report has > 200 votes, I really do want to fix it for KDE 4 ).
There are three things which can be done by various parties to improve matters:
- Developers: Take a closer look at your application and see if there are any unecessary visual borders which can be removed
- Users: Take a closer look at the applications you use and bring instances of excessive frames or poor spacing to the attention of developers
- Trolls: Please help me with the QTabWidget issue. Will sweeten the deal with beer at the next Akademy