Packages that are in Debian stable are extensively pre-tested by the users of unstable and testing. I have been an unstable user for years, and the one or two time a year things break (I update approximately weekly) and I try to report a bug, the bug has already been reported and resolved. This is what they call unstable, so imagine how well stable works.
Debian unstable doesn't have releases, so packages that make major changes have to apply cleanly to a variety of configurations (not just whatever the previous version was). Since people are testing these packages daily, any problems are resolved way before anything is marked "stable". These changes are also incremental, so instead of a testing team having to work out 100s of major changes with a six-month deadline, they are made and tested as-needed.
The end result is a system that is very up-to-date but rarely breaks. (Debian stable probably "never" breaks, but it is not exactly up-to-date either.)
