There are some things that Dropline does indeed replace. Most of these replacements are required for the sake of updating to Gnome 2.8.2, and include security patchs/optimizations/etc., and making Slackware more functional for the average desktop user. We also include our own x.org X11 build with optimizations and patches (like for fixing broken XV overlay support), and other things (like new default cursors). It is completely compatible with existing Slackware software. Some people wish that we would leave this one out of Dropline, but Slackware's versions lack several fixes and patches. It is possible to blacklist these things though, but most users don't do this. It's all a matter of preference (as there is no real reason to keep the old ones unless you require proper support with ATI's binary drivers, though ATI promises an update on Monday).
Dropline is also optimized for i686 compatible CPUs. We also believe that it is more refined and more usable than the default Slackware Gnome desktop.
If you have any questions, you can probably get a more prompt resonse from our forums at http://www.dropline.net/forums
. There is also an unofficial liveCD out there right now; with Dropline snapped onto Slackware 10. I've not used it (yet), but I am interested in seeing how it turned out. It might be a good way for users to test-drive Dropline Gnome before installing it on their machines. We'll probably post some links to a torrent within the next day or two, but we can't verify its functionality - as it is an unofficial project.
Please feel free to stop by Dropline.net if you have any more questions.