From a sitebuilder’s/IT fool’s perspective, working with WordPress is a pain in the back. It has no real MVC separation, some things can not be styled with the current hook/override approach (not enough predefined css classes), we have no ajax API at all (and if I do make one, the core WP bootstrap makes it pretty slow), css separation is not perfect (admin install.css #error-page for example, see next), hardcoded error page in functions.php, no masterpage templates (includes everywhere), no unified coding style/QA for plugins and many badly written plugins and so on.
It’s nice and cool actually – until you try to modify things.