KWin, KDE's window manager, has been around since KDE 2.0 (replacing KWM in KDE 1.x) and has grown to be a mature and stable window manager over the years.
For KDE 4, however, there were a few people rumbling about visual effects, and perhaps KWin was feeling a little envious of its younger cousins Compiz and Beryl. While these new effects have created a lot of buzz around Linux and UNIX, long-term KDE users have wished they can enjoy the effects of Compiz/Beryl while still having the tried and tested window manager that is KWin. As a result, for KDE 4, KWin has received a huge graphical upgrade, with composite and GL support.