Get Over It
A confession: I was among the many who griped publicly (and privately) about KDE 4.0's release date. I thought KDE 4.0 was not ready for release and should be held back until it was. (Hey, my wife and my dog listened to me. So there.)
Why? There were two main reasons.
First, one of the things I've loved the most about KDE is its large range of configuration options. For example, there's about a gazillion things you can do with the "kicker panel" (a.k.a. the taskbar) at the bottom of the screen in KDE 3.5.7.
Some of KDE 3.5.7's many kicker configuration options
By comparison, there's little you can do with the panel in KDE 4.0 — and, compared to KDE 3.5.7, not a lot you can do with the desktop, either. It didn't help that the betas and release candidates of KDE 4 contained even less functionality than the final version.
Second, although I've been playing with Linux distros since the late 90s, I didn't start using Linux seriously until around 2003. So the only version of KDE that I learned well was KDE 3. Changes to the KDE 3.x series seemed incremental; in other words, if you ran a live CD containing KDE 3.0, you could definitely recognize it as the precursor to KDE 3.5.8. I assumed KDE 4's user interface (UI) would be a "superset" of KDE 3.x's.
Instead, the KDE developers chose to concentrate on rewriting KDE's "plumbing," focusing (as far as this non-developer can tell) on simplifying and modernizing KDE's application programming interfaces  (APIs), and updating it to Qt 4 (Qt  is the development framework that KDE's based on). Among other things, this new "plumbing" includes a multimedia back-end named phonon ; a new way of dealing with hardware named solid ; and, most noticeably, a new user interface named plasma . As its Wikipedia article notes, "Plasma is still undergoing heavy development."
(Long-time KDE users have already noticed that, for some strange reason, none of the names for these technologies begin with the letter "k.")
Wikipedia mentions that, in KDE "x.0" releases, the developers can break compatibility with former releases. They didn't go that far, since KDE 3.5.x apps seem to run just fine on KDE 4.0. But we did see an example of the open source mantra, "release early, release often," in action. And it's worth mentioning that, while still very rough around the edges, the released version of KDE 4.0 was greatly improved compared to the betas and release candidates.
Developers as Politicians
Something else happened that was interesting. Several KDE developers, realizing the controversy there would be over KDE 4.0's unfinished state, took the time to explain what was going on. In particular, Aaron Seigo, president of the KDE foundation and the man mainly responsible for Plasma, addressed naysayers in a long blog post  titled "Talking Bluntly" (a recommended read). openSUSE's Stephan Binner wrote a couple of  posts about  the situation. As the openSUSE KDE 4 page  says:
"To make a long story short: KDE 4.0 is not and never was meant to replace 3.5.x for regular users. The main goals were porting to Qt4 and creating the frameworks to create all the things announced for KDE 4. Frameworks are unfortunately hardly visible to the user, so most things that use them, like plasmoids, panel-functionality etc., will only appear after the frameworks are in place, i.e. starting with 4.1."
(If you've got the time and bandwidth, download and listen to The Linux Link Tech Show's Jan. 2nd interview with Aaron Seigo  (link goes to a 33.8 MB audio file in ogg format). He comes across as a very intelligent, thoughtful guy, and does a great job explaining the decisions behind KDE 4. Mr. Seigo seems to have taken quite a bit of grief over Plasma, but handles criticism with aplomb. Calling someone a politician is not always an insult!)
From Kubuntu to KDE 4.0
Users of Kubuntu 7.10 "Gutsy Gibbon" were treated to a simplified file manager named Dolphin, a simplified control center named System Settings, and a web-based desktop search engine named Strigi. All three are in KDE 4.0. Moreover, Dolphin now plays the role of KDE's default file manager, and System Settings replaces the KDE Control Center.
Left: Dolphin and System Settings on Kubuntu. Right: Strigi desktop search on Kubuntu.
Dolphin, System Settings, and Strigi on KDE 4.0
KDE 4.0's user interface will seem familiar to users of former versions of KDE. The major change is the addition of so-called "widgets," or "plasmoids," to the desktop. (I'll use the terms interchangeably.) Widgets are usually SuperKaramba-like  applets, which could potentially be anything from a clock to a calculator to a system monitor. But it goes further than that — even desktop icons are widgets.
The KDE 4.0 panel (itself a plasmoid), L-R: Kickoff menu; traditional menu; task manager; desktop pager; Klipper (in the System Tray); New Device Notifier; and clock
Desktop widgets can be added by mousing over the Plasma icon in the upper right-hand corner of the screen, or from a desktop context menu. If you want to add one to the panel (such as the "traditional menu" seen above, or a trashcan), you can click and drag them from the "Add Widgets" dialog to an empty spot on the panel. You can also filter your favorite widgets from that dialog, as well as take them off your desktop.
Messing around with widgets (click for larger image)
Widgets can be rotated, made larger, configured (if there are any configuration options for that widget), and made to quit, via small icons in the frame surrouding them that shows up when you mouse over them. You can lock them to the desktop, which hides the frame (which is handy, because at this point, sometimes the frame doesn't go away when it should).
It would seem that the KDE developers wanted users to be able to differentiate between the web browser (Konqueror) and the file manager (Dolphin), instead of having the confusion of having Konqueror perform both functions. Although you can still use Konqueror as a file manager (by pressing Alt+F2 to open KRunner, and entering the command "konqueror --profile filemanagement" as before), for now, Konqueror uses Dolphin as its file management engine. (What's missing in both is a tree view in the right-hand pane. Hopefully that'll come back soon.)
Dolphin seems to have borrowed from GNOME's Nautilus file manager and from Mac OS X in its design. Of particular interest: the "breadcrumb" bar across the top, between the toolbar and the file window. Click on a ">" symbol to get a vertical listing of directories. The "breadcrumb" bar can be interchanged with a regular editable location bar with a mouse click. Also of interest: the new "columns" and "split" views (which sort of make up for the lack of a tree view in the right-hand pane).
L-R: Dolphin with "breadcrumb" bar; Dolphin using column view; Konqueror in file management mode
KDE 4.0's new default document viewer, Okular, can open up a huge variety of document formats. Gwenview is KDE's default image viewer. Konsole, KDE's terminal application, has a subtly revamped feel. (I could swear the line spacing is just a bit larger than it was in KDE 3.5.x; a pleasant surprise.)
L-R: Okular's list of supported formats; Gwenview in browser mode; Konsole's "profiles" settings
Games in KDE 4.0 now usually use scalable vector graphics, for a much nicer look. There are some interesting new applications, including Marble, a searchable "desktop globe;" and KPhotoAlbum, which is apparently complicated enough that the new user's offered a 10-minute series of introductory videos. As of this writing, KOffice 2.0 is still under development, and while some parts of it work, others (such as KWord) instantly crash.
L-R: Games in KDE 4.0; the Marble desktop globe; KPhotoAlbum; the Amarok audio player (version 2.0 "pre-alpha")
More Eye Candy
KDE 4.0's default theme, named Oxygen , comprises a new set of default sounds, a new, more photo-realistic set of icons, a windows decoration, or "windec" (the part that surrounds your application windows), and a widget style ("widget," in this sense, meaning buttons, scroll bars, progress bars, etc.).
(For those keeping score, most of these screenshots use Plastik as the windec and (the new) Plastique as the style. You still get to choose.)
The Oxygen style
If you've got a graphics card capable of displaying compositing effects  (such as you'd see with Compiz Fusion ), you'll find that KDE 4.0 includes many of its own. (They're turned off by default, but can be switched on from System Settings > Desktop > Desktop Effects.) Some of the effects are quite useful, such as the one that darkens the window underneath an active dialog box; and some are just for fun, such as the Explosion effect that makes windows that are closed, explode into pieces. Be advised that, at this point, turning on desktop effects can make other applications unresponsive, cause general flakiness, and make the entire UI unresponsive if you use an OpenGL screensaver.
   
L-R: Desktop effects settings; an Alt+Tab "box switch" window switcher; a "desktop grid" virtual desktop chooser
Finally, KDE 4.0 also comes with some gorgeous wallpapers .
KDE 4.0's release has been a learning experience, at least for me. KDE 4.0's got a long ways to go before it's ready to replace KDE 3.5.x. (For example, my desktop icons have suddenly stopped working!) But one thing is for sure: improvements (which is what you'll call them if you approve of them) and changes (which is what you'll probably call them if you don't) are occurring rapidly. (It seems like openSUSE releases new KDE 4 packages on an almost daily basis.) As far as the prior complaints go — fuggeddabouddit. Time to move on. I'm looking forward to seeing what KDE looks like, six months from now, and longer-term, years from now.
— Andrew Heil, aka "eco2geek"
For More Information:
- openSUSE's "KDE Four Live " CD – try KDE 4.0 without installing it on your computer
- The official KDE 4.0 release announcement  (with information on getting it via various Linux distros)
- A KDE 4.0 walk-through 
- More KDE 4.0 screenshots from The Coding Studio