* Reliance on ad-hoc hand-coded SQL.
* A system that registers psuedonymous Internet commenters in the same user table as Wordpress administrators.
* Mishandling of internationalization.
Yes, Wordpress is also the most popular blogging platform, and yes, it's also much more dynamic than its competitors, but phpBB and vBulletin seem to have had better track records than Wordpress have. It's just not very secure.