Wordpress is a great blogging tool, the capabilities of which for blogging currently surpass that of MODx’s default snippets/plugins.
MODx is a Content Management Framework that allows you to build anything you’d like much more quickly and simply than traditional build-from-scratch or hack-to-death methods. More simply, it should be possible to build Wordpress’s functionality in MODx, but you’ll never be able to replicate MODx with Wordpress. (Well, at least not without boatloads more work, the end result which would not really resemble WP at all...

) Too boot, you’d still have a clean upgrade path with MODx when you make custom applications assuming you use it’s API and event system.