I've dived in the Wordpress codebase a few times, I think it wouldn't be very hard to rewrite major parts. However, I don't think you can fix much without breaking tons of plugins, themes and 3rd party tools that integrate with Wordpress.

And if you are going to break the plugin ecosystem, might as well use Habari, which is much nicer: http://habariproject.org/en/

Too bad all the available plugins fit in a single page: http://wiki.habariproject.org/en/Available_Plugins