The splitting of the distros like this is really more to do with how much cruft you have to download and install on the system. There's really no difference between the server version and the desktop versions except for which software packages are bundled on the media (DVD) + and what is installed by default when you use the particular versions.
As an example you could easily install CentOS using the server or minimal editions and then once the system is up and functioning, run a command such as:
$ yum groupinstall '..Desktop Environment..'
to install all the packages required to make it a fully functioning desktop edition.
As to what else is missing, well that really depends on each distro. With server distros, they're usually trying to cut out software to make the download smaller and more manageable. So things like a web browser, LibreOffice, and anything GUI related will typically not be included in a server edition. They'll also omit non-GUI software like compilers, such as gcc, and a whole host of other software, which really have no business being included in a truly server install.