Here are some ways the various distros differ:
1) Whether the distro is free or commercial and uses "Free Software"
2) Number of packages available without looking for other sources of software. Debian is the biggy here with over 15000 apps.
3) The way updates are handled.
4) Security policies.
5) Package management systems
:( there are advantages and disadvantages to all methods)
The following are some examples of package management systems implemented by Unix-like operating systems:
* RPM, the RPM Package Manager. Invented by Red Hat, but now used by several other Linux distributions. RPM is the Linux Standard Base packaging format.
There are many higher level tools that use the RPM packaging format, simplifying the process of finding, downloading and installing packages and their dependencies, including
o YUM, used on Fedora Core.
o up2date, used on Red Hat Enterprise Linux. Although designed to talk to Red Hat Network, up2date can also source packages from yum and apt for RPM repositories, as well as local and directories.
o YaST , as used on SuSE Linux distributions.
o urpmi as used by Mandrakelinux/Mandriva
o APT for RPM.
* dpkg, used originally by Debian GNU/Linux and now by other systems. The .deb format used by dpkg was the first to have a widely known dependency resolution tool, called APT.
* Portage/emerge, used by Gentoo Linux and inspired by the BSD ports system.
6) Is it going to be around or is it a new distro.
7) "Comparison of Linux distributions"
8) The file system used. "Comparison of file systems"
9) Window manager it uses. Window Managers for X
10) Desktop environment
11) How easy it is to install and use. How are programs installed and updated.
12) Does it have what "you" Need.
13) The documentation and support available.