The Agile Manifesto was a game changer, and the 12 principles are solid. I often go back and re-read them to make sure I really believe we are following Agile practices at the IBM Jazz Project. There is always the risk that some teams focus most on the practices that "protect" the development team or make the life of the developer easier, and gloss over the practices that are important for the stakeholders and essential to achieving the right outcome. I think one of the hardest for teams to really internalize is "Welcome changing requirements, even late in development".
Agile Manifesto Review
Dave Thomson
IBM Jazz Project
The original review was posted on agilemenifesto.org.