Here is a list of packages, present in both Salix and Slackware repositories and the reasons Salix is using a different build:

Salix 15.0

elilo: Minor tweaks to eliloconfig.
lilo: Salix artwork. Minor tweaks to liloconfig.
mozilla-firefox: Using the latest ESR version.
sysvinit-scripts: Tweaked for boot speedup. Colorized boot messages. Autostart feature for every executable file in /etc/rc.d/.
xfce4-whiskermenu-plugin: Newer version, patched for using proper group icons.

Salix 14.2

elilo: Minor tweaks to eliloconfig.
hplip: No QT GUI.
lilo: Salix artwork. Minor tweaks to liloconfig.
mozilla-firefox: Using the latest ESR version.
sysvinit-scripts: Tweaked for boot speedup. Colorized boot messages. Autostart feature for every executable file in /etc/rc.d/.
v4-utils: No QT GUI.
wpa_supplicant: No QT GUI.

elilo: Minor tweaks to eliloconfig.
hplip: No QT GUI.
lilo: Salix artwork. Minor tweaks to liloconfig.
mozilla-firefox: Using the latest ESR version.
sysvinit-scripts: Tweaked for boot speedup. Colorized boot messages. Autostart feature for every executable file in /etc/rc.d/.
v4-utils: No QT GUI.
wpa_supplicant: No QT GUI.

gst-plugins-good: Many more plugins, that are essential for gstreamer actually being of any use, are included.
hplip: No QT GUI.
lilo: Salix artwork. Minor tweaks to liloconfig.
mozilla-firefox: Using the latest ESR version.
p11-kit: Built with support for engine_pkcs11.
sqlite: tcl support is broken in the Slackware package (32-bit only)
sysvinit-scripts: Tweaked for boot speedup. Colorized boot messages. Autostart feature for every executable file in /etc/rc.d/.
v4-utils: No QT GUI.
wpa_supplicant: No QT GUI.

hplip: No QT GUI.
libvorbis: Using the aotuv version instead, provides better quality encodes. Fully compatible with the official library version.
lilo: Slackware package repackaged with different artwork. Minor tweaks to liloconfig.
mozilla-firefox: Using the latest ESR version.
shadow: Patched the Slackware package for fixing a bug where the environment was not set correctly with su.
sysvinit-scripts: Tweaked for boot speedup. Colorized boot messages. Autostart feature for every executable file in /etc/rc.d/.
wicd: The Slackware package is broken.
wpa_supplicant: No QT GUI.
xfce: Mount vfat devices with thunar using utf8 by default.

cups: Patched for behaving with gnome-vfs.
gst-plugins-good: The Slackware package has no support for using audio/video with a webcam.
libvorbis: Using the aotuv version instead, provides better quality encodes. Fully compatible with the official library version.
lilo: Slackware package repackaged with different artwork. Minor tweaks to liloconfig.
mozilla-firefox: Using the latest ESR version.
pkgtools: The pkgtools package is available, but Salix is using spkg as default, however the original pkgtools are included even in the spkg-pkgtools package. Spkg is written in C and is much faster, see their website
sc: The slackware package doesn't really work (x86_64 only)
shadow: Patched the Slackware package for fixing a bug where the environment was not set correctly with su.
sysvinit-scripts: Tweaked for boot speedup. Colorized boot messages. Autostart feature for every executable file in /etc/rc.d/.
hplip: No QT GUI.
wavpack: Newer version.
wicd: The Slackware package is broken.
wpa_supplicant: No QT GUI.
xf86-video-intel: Offer both 2.11.0 and 2.9.1 versions of the driver. The older driver is needed for PCs with intel cards that fail to work with KMS with kernel 2.6.33.4.
xfce: Patched mousepad for fixing a bug with the Find text tool.

libvorbis: Using the aotuv version instead, provides better quality encodes. Fully compatible with the official library version.
lilo: Slackware package repackaged with different artwork. Minor tweaks to liloconfig.
mozilla-firefox: Using the latest ESR version.
pkgtools: The pkgtools package is available, but Salix is using spkg as default, however the original pkgtools are included even in the spkg-pkgtools package. Spkg is written in C and is much faster, see their website
shared-mime-info: Support for xz compressed files.
sysvinit-scripts: Some code added to support splashy at bootup. Autostart feature for every executable file in /etc/rc.d/ added.
hplip: No QT GUI.
wavpack: Newer version.
wicd: The Slackware package is broken.
wpa_supplicant: No QT GUI.
xfce: Newer exo that fixes several problems.
xfce4-power-manager: Newer version.