Not to be contrary but:
# Must be platform independent (at least Linux/Win)
# MySQL may be a requirement, but I'd be just as happy with flatfile
Extensible & easy to use are reasonable requirements, but who cares what kind of database or OS you need to run it on, as long as it's not hideously expensive or unstable. Why not run it on ruby/freebsd/postgres (just as an example)?.
I can't really give you an answer to your general question though, since every single generic CMS I've looked at sucks in more than a few ways (anything from generating hideous markup and being hard to work with as an end user to being far too inflexible and/or difficult to upgrade with custom patches/changes). And I don't really care about the implementation language (as long as the language doesn't preclude it from being reasonably cheap and easy to deploy and extend).