A window manager draws a box around your apps with a title bar and buttons to iconify, maximize, shade, whatever. Or keys to do so. It generally gives you a way to launch apps such as a menu or command line or desktop icons. It generally has a directory of a handful of small plain text files you can edit to configure it. It's generally one or a few binaries.
What more do you need?
An IDE is a huge mammoth thing that drains resources so that it can draw all kinds of widgets all over the place and lock you into using the KFile Manager and GText Editor and making you load a KGDefault GKBrowser. It's slow. It comes with a zillion binaries and thousands of indecipherable config files that you aren't supposed to touch because everything must be done through hundreds of clicky dialog boxes. None of this is remotely necessary to shoving a window around.
As far as the graphical login, I feel the same way about ?DMs as I do Knome. - Rip it out, boot into text mode, type 'startx'. Want to launch something else? Change the 'exec' line of ~/.xinitrc to something else. Beautiful, ain't it?
(Seriously - I dunno how to change the clicky thing but there is a file or directory of them (I forget) that you can edit to force stuff to appear on the menu... if you can find the file amidst all the junk.)