Version:            (using KDE KDE 3.5.5)
Installed from:    SuSE RPMs
OS:                Linux

Currently handling multiple documents/tabs and sidebars work differently in Kate, Kile, Quanta+, KDevelop, KOffice (which actually can't handle more than one documents in one window) and Konqueror.

 - Konqueror has tabbing. It is acceptable, although some improvements could be made (e. g. dragging a tab without pressing Ctrl should move it instead of copying it). It features a New tab and a Close tab button. The tabs also have a popup menu.
 - Kile also has tabbing but somewhat differently: there is no New tab button (no problem, there is New document instead) and tabs can be closed with clicking on the icon after a delay, as there is no Close tab button on the right. Tabs do not have a popup menu.
 - Kate also handles multiple documents at the same time, but in a quite useless way. It has a Documents sidebar instead of tabs, in which the documents are listed in a column, so it occupies much more place on the screen. (Especially if few documents are open.) It also has a Tab bar extention, but it can only do switching between documents - no popup menu, no Close button etc. and if you have more documents open than how many fits on the screen then it requires two clicks.
 - KDevelop has a quite well configurable UI, and Quanta as well, but not the same system.
 - KOffice components do not feature MDI at all, they use top level windows.

This means that if you configure the MDI/tabbing in one application, and get used to it, it may be completely different in an other app. And you have to suffer from useless MDI in some applications (e. g. Kate), while it is already implemented much better in other apps.
 
 The same applies to sidebars: just try to open a Konsole inside Konqueror, Kate, Kile and Quanta and see the differencies.

I think all these applications should use a common MDI and/or sidebar system and it should be configurable in the Control Center.