Cons:
1) Many of the Debian-specific commands and subsystems require time and reading to use - they are targetted at experienced users. In contrast, most other distro's unique components are wizard-like GUI programs whose operation is fairly obvious once you know where to find the program in the first place. You can't get the most out of tools like aptitude or make-kpkg without sitting down and reading.
2) The stable branch moves too slowly for desktop systems. The unstable branch is up to date, but can require knowledgeable management when a component undergoes major changes, which may happen at any time. Testing can work well much of the time, but can occasionally have portions (like GNOME) stuck in screwed-up states for significant periods of time because of key packages being held back.
Pros:
1) Best quality packaging and configuration system out of any distro. Subsystems and policies are thoughtfully designed - it is extremely rare to see anything in Debian that feels like a "hack job".
2) Huge selection of software.
3) With appropriate management, can work well in a huge range of environments - from minimalist, CLI-only embedded servers to cutting-edge desktops.
Debian is, IMO, still far and away the highest-quality distro out there for the largest range of purposes. It takes a while to understand how Debian works and why it works that way, but once you do it's hard to make yourself use anything else.
