Kwin, the KDE window manager, includes three dozen desktop effects that you can use if you have at least a couple of megabytes of RAM and a video driver with 3-D support. You can choose the effects you want to use by selecting Main Menu -> Favorites -> Configure Desktop -> General -> Desktop -> Desktop Effects.
Like other desktop effects, KDE's vary from practical ones like the Magnifier and Zoom, which can improve desktop accessibility, to the convenient, like Dialog Parent, which darkens a window when a sub-window pops up, to pure eye candy such as displaying virtual desktops on a cube or having windows fall to pieces when they are closed. Many of the effects have their own settings to fine-tune how they start and operate.
You also have the option on the Screen Edges tab of activating effects by moving the mouse cursor to one of eight points at the edge of the screen. However, currently, you are limited to assigning six effects to the edges, most of which are more decorative than practical.
The distributions that I've seen are set by default to use OpenGL as the compositing type. However, you may have to go to the Advanced tab and select XRender to enable some of the effects.
KDE 4 introduced the Kickoff menu. This menu features a search field on the top, and five views: the default Favorites, and Applications, Computer, Recently Used, and Leave. Kickoff shows only one menu level at a time, providing right arrows on the right to descend a level, and left arrows on the left to ascend a level.
Many people consider Kickoff an improvement on the old accordion-style menu, in which all menu levels are visible at once, spilling out across the desktop. However, just as many seem to dislike the change.
If you are among those who dislike Kickoff, you can right-click the menu button and select Switch to Classic Menu to get an accordion-style menu.
Alternatively, if your distribution includes the package, you can install Lancelot, an alternative menu that displays several menu levels at once, but confines all levels to a single window -- a sort of compromise between the Kickoff and Classic menus. Lancelot's top level is divided between Favorite and Applications columns, and includes Documents, Contacts, and Computer views, with logout options along the bottom.
To install Lancelot, right-click the panel, and select Panel Options -> Add Widgets -> Lancelot Launcher. The icon appears on the far right of the panel, so you will probably want to right-click the panel a second time and select Panel Options -> Panel Settings, then drag the icon to the left side of the panel. You can right-click on the Lancelot icon to further customize the menu.
No matter which menu you choose, you can right-click and select Menu Edit to alter and re-arrange contents. However, you can only edit the applications menu. Top-level views, such as Favorites, are still uneditable.
On most operating systems, the system tray can quickly fill with icons, many of which you don't care about and have to pass the mouse over to know what they are. Windows will hide little-used icons, but KDE allows you to right-click on the tray and set the icons to hide for yourself. If you click the arrow on the left of the tray, you can view the hidden icons if necessary.
The task manager is the list of open windows. In earlier versions of KDE 4, the task manager was rather basic, but in 4.2, it comes with a variety of options. Right-click on the task manager, and you can set how many rows it uses to display windows, how or if windows are grouped and sorted, and whether windows from every desktop are available.
Another useful way to customize the task manager is to select Main Menu -> Favorites -> Configure Desktop -> General -> Desktop -> Desktop Effects -> All Effects -> Taskbar Thumbnails. This desktop effect will display a thumbnail of a window in the task manager when the cursor rests on it.