File icon previews should be turned on for all icon types by default because this is 2014 and to see a generic icon where a miniature .pdf thumbnail could be, or a miniature office document page could exist is *unfriendly* for such an advanced OS as KDE. Currently, only image types are shown by default for previews.

In contrast to Kubuntu, there is Windows 7/8, Mac OS, and Ubuntu which all show icon previews of file types by default in their respective file explorers.

So clicking on the Folder wrench icon -> Icons tab -> "More Preview Options" button tab, all the items should be checked by default. They also should be checked by default when any new file type is added to the system [if the others were also checked by default], such as recently Marble added new KML types to my system that were disabled for preview even though I have manually checked all the other types. 

One thing is broken with file previews that will soon become evident if this request is fulfilled: having the "Text Files" and the "Desktop Files" types enabled makes .desktop files show an ugly _text preview_ of the .desktop internals instead of the intended .desktop icon.

Thank you for listening. I've installed Kubuntu on a fair number of machines and there's some things that I've had to do over and over just to get an experience that my friends are expecting. Making all file previews display by default would be an easy change the perceived friendliness of KDE.

This would make sense to go hand-in-hand with having file previews turned on by default in Dolphin as well (bug https://bugs.kde.org/show_bug.cgi?id=338492).