I’ve been an enthusiastic KDE user since the Red Hat Linux 5.0 / KDE 1.x days. The best Linux desktop experience ever was provided IMHO by the KDE 3.5 series.
KDE 4.x was a complete disaster for me. I do realize that the 4.0 series were an experiment on their loyal user base, sorry, I mean an early release aimed to gather user feedback rather than provide the ultimate Linux desktop as we’ve come to expect after the excellent 3.5 series. I know that, I did read all the right blogs, etc. So I’m willing to forgive KDE 4.0’s countless bugs and tons of missing features.
What I can’t forgive is that while there have been gradual improvements, KDE4 remains trouble-ridden to this day. Even two years later the current stable release (4.3.5 – the last in the 4.3.x series according to kde.org) still has some glitches that annoy the hell out of me.
My number one pain that nags me every day is that the damn taskbar rearranges itself every time you switch virtual desktops. I’m using the Manual task arrangement option, and I expect when I return to a virtual desktop to find the same tasks in the same order I left them. Come on, guys, WTF?
Amarok is the best audio player application that I’ve ever seen. On all platforms. But it took forever to port it to KDE4. And the bugs I’ve encountered with it over the last two years have frustrated me to a point where I’m using rhythmbox under KDE 
A few days ago Amarok would not even output any sound under Fedora 12 at all. It used to, but some update broke it. Fortunately, right now the latest update works. Still, I don’t remember such brutal issues with KDE3/Amarok1. An audio player that fails to output any sound… come on, guys, WTF?
And even now with the latest Amarok 2.2.2 and KDE 4.3.5 I’m having the ages old problem where playback will sometimes stop after the current track. I have really tried but I can’t figure out how to reproduce this reliably.
But the good thing about Linux (and other Unix-alikes) is that there are plenty of desktop environments to choose from 
BTW, when I’m talking about a desktop environment, I’m including applications which are not strictly part of the respective desktop environment. Basically I’m counting all QT/KDE apps on KDE’s side, and everything that is GTK on GNOME’s.
GNOME has always felt too simplified, rigid and plain, compared to KDE’s excellent customizability and countless bells and whistles. But to my amazement after using it for a while it grows on you. The various GNOME apps (terminal, audio player, rss reader, etc) have always seemed inferior to me compared to their KDE counterparts. But less so in more recent versions. For example, rhythmbox now has a context plugin to display info about the currently playing band, song lyrics integrated right into the player window, and while it’s not an Amarok yet, it is inching closer and closer.
I’m just getting started and it already seems this rant is getting a bit too long
Here’s a brief summary about the things I like/dislike about both environments:
KDE niceness:
- Many KDE applications are still better imho compared to their GNOME counterparts, e.g. amarok, konsole, akregator, ktorrent, kdesvn, kompare, konqueror (file manager), etc.
- Applications like konsole remember open tabs after logging out and logging back in.
- The start menu is searchable (the only undoubtedly useful feature of the great KDE 4 re-imagining)
- (minor) The plasma widgets are nice, but not a decisive feature for my choice of a desktop environment. I usually have plenty of windows open on all virtual desktops so I rarely see my desktop
- (minor) I like developing QT UIs a lot better than developing GTK/Glade.
KDE brain damage:
- (major gripe) The tasks on your taskbar rearrange themselves when you switch virtual desktops (even when the ‘Arrange Manually’ option is selected!) – update: this is fixed in KDE 4.4, see Rex Dieter’s “Fedora 12 KDE 4.4.0 Remix“
- Amarok still has issues.
- (minor) Starting a KDE app is noticeably slower under non-KDE desktop environments. Starting additional apps is better though.
GNOME advantages:
- It clearly shows that significant amounts of thought and effort were spent on cleaning up and streamlining its UI. GNOME provides the Mac experience on Unix systems
- GNOME’s 3D desktop is the best
- I really like the GNOME weather applet
- (minor) GTK is the native widget library for a number of critical apps for me: Firefox, Google Chrome, Miro, Pidgin, Eclipse. No significant performance hit is observable when running all those under KDE though.
GNOME disadvantages:
- (minor) I could never get used to the spatial Nautilus feature. I usually have too many Nautilus windows open with too many items in them. Luckily the file browser mode (it has tabs!) saves the day.
- (minor) The default configuration with two panels – one on the top (with start menu, systray, but mostly empty) and one at the bottom (task bar, virtual desktops, etc) is a HUGE waste of screen space, especially on a wide screen. Luckily it’s quite easy to merge the two panels into one and still have a comfortable experience, even on a 4:3 display, so this is hardly a showstopper.
Final Verdict
I’m still a huge KDE fan…
But there’s no way I’m using KDE until the taskbar rearrange bug is fixed (4.4 is due in ten days…)
In the mean time I will feel comfortable in my customized GNOME environment