Most notably: Slackware does not include gnome
Slackware does include KDE, Xfce and Fluxbox.
It's better to copy xinitrc sample files to your user's /home/your_user rather than messing with the ones in /etc
$ cp /etc/X11/xinit/xinitrc.xfce4 ~/.xinitrc
You do not need to do this if you change the default run level and start a display manager - see the section on "graphical login" in the beginners guide: http://docs.slackware.com/slackware:beginners_guide
If you do a full install of Slackware, you will have xorg, all of the above desktops/window managers and the KDM and XDM display managers installed by default.
If you want something else, e.g. openbox, you will have to build it from source. The best way to do this is to download the source tarball and build script via http://slackbuilds.org/
- this will generate a Slackware package which is always preferable to manual builds and installs (autotools, etc).
For gnome users there are packages available here: http://www.droplinegnome.org/