Um... that is a loaded question.
ABF you wanna field that one?
My opinion is that the distros really come down to what software is installed on them. Some of the distros like Gentoo or Debian(Or Debian Based ones) will be idientified in part by their software management solutions, Portage or APT in the case of the two above.
However all of this comes down to, linux is linux. The distros all run linux, and thus any distro should be able to run any linux software, though some have proprietary software on top of Linux to 'help' with various aspects of things. Suse with the Yast software is a good example of this. Sometimes this does help, sometimes it beccomes a serious hinderance. Personally I believe it is important to learn the config files, at least the basics before using GUI based things for this nature as you will know more and have more control/flexibility than the GUI typically offers.
Some distros are known for installing everything and the kitchen sink, or being tuned for a certain window manager, or other quirks about it. Personally I prefer a slightly lighterweight distribution that I would install my own software on top of myself, that is why I tend to reccomend Ubuntu for newcomers, it is relatively lightweight, typically a good installation, but with all the config files that you can play with you can still learn things.
Suse, Madrake(Mandriva), Fedora, and the like I tend to think install to many things to be worth it. My experience has been less than plesent when compared to other distros, and they tend to be slower by default than some other distros, though Fedora especially this can be remedied to some extent in.
Um other things, well it really comes down a lot to personal choice among other things. Obviously I support Ubuntu and Gentoo myself, others here support Fedora, or Mandriva, or Knoppix(Live CD based distro, meaning it is designed to run off the CD), or one of a million different things. Primarily personal choice for the most part.