Normally PHP isn't my favourite language, but Textpattern looks good, the only security hole found was only found recently in a 2½ year old gamma release. Impressive for a PHP application, and probably in no small part due to Textpattern not including XML-RPC where PHP has historically had a lot of problems.
And I really like Textile, the default “markup” used for writing in Textpattern.
The worst thing I can say about Textpattern is its lack of support for themes. It's just too hard to work with, although there's a feature request about it, so it will probably improve.