I don't actively use gnome or kde (although I tend to install them when a new main release comes to Sid to check out what's new) so I cannot give any detailed info, just something I can recall from my earlier experiments with gnome & kde.
1) By "gnome launch bar" you probably mean the gnome panel (that is by default located on the top of the screen). Have you tried right-clicking this panel and following the options given in the drop-down menu? I personally quickly removed the other panel from the bottom of the screen and added pager, launch icons and other useful stuff to the main panel on the top of the screen.
2) The kde menu is very configurable -- you can easily add and remove items (right-clicking either the start button or the kde panel gives you the configuration options, as far as I can recall). There's also a special utility in kde that scans your system for unused program executables and this utility also gives you the option to add any of the found applications to the kde menu. The launcher for this utility is somewhere in the kde menu -- try to find it.
3) Defining the default desktop varies depending on the method you use to log into your Debian box. If you log in from the command line, you need to create an .xinitrc file into your home directory to define the default desktop. If you use xdm to log in, the file to create is .xsession, which needs to be executable. (I think that there's a GUI utility for changing the default desktop for xdm, but I don't remember its name.) You can also use kdm or gdm and these have their own ways to configure X sessions.
And, to make this all just a little bit more complicated, the method used to configure X sessions for kdm and gdm appears to change depending on the kde & gnome version you use. You seem to be using Sarge, which has kde 3.2 -- I'm not sure if gnome 2.6 is yet in Sarge.
Perhaps the people who use kde/gnome actively can give you better advices.
Edit: For wmaker fans there's also Login.app, the coolest lookig graphical login manager I've seen -- but I might be a bit biased here because I find wmaker the coolest looking wm I've used, and yet many people don't like wmaker at all.