I'm also interested in its variants, like kubuntu-desktop. It appears to include GNOME (or KDE) and a bunch of other things. But it's quite large – is there a breakdown of what is included in ubuntu-desktop and exactly what those items are/do?
Ubuntu – What exactly is in ubuntu-desktop
The ubuntu-desktop (and similar) packages are metapackages. That is, they contain no data (besides a small documentation file in the case of the *-desktop packages). But they depend on dozens of other packages that make up each of the Ubuntu flavors.
You can see a complete listing of each package's dependencies on packages.ubuntu.com. A quick search lists these metapackages.
A more detailed explanation of metapackages and a list of some more useful metapackages can be found here.