Version:           unspecified (using KDE 4.4.5) 
OS:                Linux

There is currently no easy way to make an application the default application for all of it's known file types. The only way to do so, requires that you manually set each mimetype to the application.

Two such examples of where this is useful are: 
1) Handbrake (a movie converter app) currently sets itself as default for all movie files on fedora. Most users however probably want their default to be a movie player. 
2) A user who previously preferred Dragon may wish to switch to VLC as the default easily. 

Reproducible: Always

Steps to Reproduce:
Using (1) As an example
1) Dragon Player and handbrake are installed, Handbrake is the current default. The user doesn't want to uninstall handbrake, because they need it for their portable media player

Actual Results:  
1) System Settings
2) Notice, Default applications WONT work here, because it only deals with handlers
3) Go to advanced, file associations 
4) Expand Video.
5) Go through EVERY entry, select Dragon Player, move up, and apply.
6) Note, this doesn't scale well.. The more powerful an application is, the more difficult it is to set it default for everything. 

Expected Results:  
The better way is:
1)System Settings
2) Go to default Applications 
3) (New) Switch to the NEW "set preferred application" settings, from the basic handlers
4) (New) A listing of the applications from the menu are displayed. 
5) (New) Select your preferred application from the list
6) (New) Click set application as default (in this example, Dragon player). 
7) (New) KDE internally sets Dragon player as primary application for all compatible mimetypes. 
8) Note, regardless of how many mimetypes the app supports (could be hundreds), there are only 2 or 3 clicks, as opposed to hundreds potentially. 

Microsoft introduced this in Windows Vista(?), and it is used quite often: http://www.nirmaltv.com/2009/01/31/how-to-set-up-default-programs-in-windows-7/