Debian and Redhat have been fighting a long battle since long. This is due to their implementation of linux and the philosophies they follow.
Debian is a community based distro, or should i say parent distro. Its main philosophy is to use softwares that are under the GNU GPL only. So you will never find a debian rolling out with KDE. The child distros or derivatives may, for example Kubuntu and Knoppix.
Red Hat on the other hand is a company. They are here to make business and so they are ready to have softwares that are non-GPL clubbed with their distro. They can even add other open source projects which follow other open source license.
But difference still exist in many other aspects. One major aspect is that of packaging. Redhat uses the popular RPM(RedHat package manager) where as Debian has their dpkg(Debian package). Both distros now have their repository based installers like yum/up2date for RedHat-Fedora families and apt-get for all debian based distros. You can't install a debian package easily on a Redhat nor can u easily install an RPM on a debian. There are softwares that can help you with that. Softwares like Alien. But they are not as acceptable.
Still all of us agree that RedHat and Debian are the biggest players of Linux market. This battle will go on for ever until if one day both decide to merge which probably is just a dream..