After customizing KDE in general, you will want to customize its window manager, called "Plasma", and its panel.
Again, these are merely my choices. I invite you to experiment and find the settings you prefer.
* Panel > Show Panel Configuration
Position: top
Panel height: 30 pixels
Good UI ergonomy implies that the same concepts are used in similar way throughout the desktop. Many applications have a row of tabs, where clicking on the tab shows separate content for each tab. And the row of tabs is at the top. Why should it be at the bottom? Just because some Microsoft "engineer" in 1995 thought it would be cool to have pull-down menus that extend to the top like gravestones??
(Some editors even use rows of tabs at the top in some places and at the bottom in others: In the Eclipse IDE, open files tabs are at the top, and file views in the XML file editor are at the bottom. What a mess!)
* Panel > Configure Icons-only Task Manager
Appearance: General: Show small window previews: Disable.
Behavior: Clicking grouped task: Shows textual list.
The previews disturb me while I'm moving the mouse near the panel. And I have that many application windows open, that a separate icon on the panel for each one (only a few pixels wide) would make the panel virtually unusable.
* Time/Date widget > Configure Digital Clock > Appearance: Show date: Disable.
I don't need to spend room estate on something that stays constant over the entire day.
* Add widgets... > Network speed
The network speed widget tells me a lot about the processes running on my machine: when a large download is finished, when synchronizing with my NAS is still ongoing, whether my DNS lookups are failing due to bandwidth issues or due to wi-fi trouble, and so on.
Some more widgets exist, but the network speed widget is by far the most useful for me.
