I have written previously on some of the CMS that I have used. What I have yet to see in any of them, be it open source or closed, written in any language that you care to name, is the CMS that I want. This rant is my view of what a CMS should be.
All websites have a simple structure, whether we are talking about a chaotic mega-entity like Wikipedia, or a little personal home page. I should be able to lay out, in arbitrary depth, my website. Moreover, I should be able to do this in a declarative way. No hacking. Just, let me arrange my data. Websites will remain largely hierarchical (show me one that isn't) just by the nature of things. Our file systems are hierarchical. You enter a website at a predefined point (though Google and bookmarking allow us to jump to more random places in the tree) and move on from there. This is not going to change any time soon.
A website may fill any number of functions. Blog. Collection of pages. Wiki. These can change at a moment's notice. I want to be able to compose these functions as I see fit. Drupal, Joomla, & friends are miserable in this regard. Their ecosystems have ecosystems. Rather than building small and beautiful, allowing the user to compose the results, we get tangled jungles within jungles.
Templating should be simple. I'll lay it out, you fill it out. Most CMSes make far too much distinction between different kinds of blocks and content. I don't want to care. At all.
All of this should be malleable at a moments notice.
A SOAP interface for handling that text-based content would be nice.
In short, it just shouldn't be this convoluted.