Joomla!? I have virtually no experience with it, but it meets at least three of your requirements: It is open-source, it is highly customizable, and, thanks to DreamHost’s one-click install system, even a preschooler could install it!* As for whether it is “quick,” do you mean quick to set up, quick to configure, or quick in serving pages? The former is presumably so, but I really can’t say as far as configuration and end-user speed are concerned. Similarly, I won’t pretend to have any idea how scalable Joomla! is.
*Not a legal guarantee
Very little to do with either emus or farmers!