Comparing MySQL and Postgresql

Mr Poet would like to start a discussion on the following: "I am an AVID user of Postgresql, over the past 6 months I have seen more and more about MySQL. I have done a little research and outside of MySQL being extremely fast I have not seen any reason why people are so excited about it. This is not a slam on MySQL. I just don't understand the reasons. Could we have a technical discussion as to why?" That's a good question. What are the strengths and weaknesses of both programs. Are there areas of deployment where one is more appropriate than the other, or are these programs fairly interchangeable?