A constantly usable testing distribution for Debian

I definitely like the ability to upgrade software to the next great version, instead of being stuck with old and outdated one, however secure and stable it might be. This is especially valid for typical user-level applications, where features are important such as OO.org, firefox, eclipse, gimp, games, multi-media, version control clients etc. These usually are the leaves of the dependency tree and can easily be upgraded. If they break, they don't affect other packages and can be replaced simply with new versions.
It is usually less important to be up-to-date with the core packages such as kernel, gnome, glib, dbus and such. For those it is more important to have stable versions. As long as it is possible to use the latest features of my user programs for work or whatever, stable core is a good core.

That sounds like a good idea for a distribution - cutting edge user programs (with several versions to choose from) and a stable, hardened system core.