Expressionengine-Yah, its very, very powerful but, you still need a lot of technical knowledge to make it work. Also, it needs to be Saas to make it worth while and a better Drag and Drop WYSIWYG interface. I think it is on the right track though.
As far as comparing it to Drupal or Joomla, I dont think you really can. Its more of a tool than an operating system like the open source systems. My issues with open source platforms are that you never know if and when you will get a feature or a bug fixed when you need it. I have customers who were waiting for several months because some dude in South Africa was supposed to work on something and never did. I also have clients worried about security with open source.
On the CMS front, Expressionengine is not a pure CMS but more of a Building and Managing platform like Sitemasher or other integrated solutions trying to change the paradigm. I see this as the way to go but, no one is completely there yet. I would anticipate we will see some new Web apps that provide a complete tool for Building, Managing and Optimizing sophisticated websites without programming soon enough....it just has to get easier.
CMS Sytems- The whole reason CMS systems exist is because of the way we build websites to begin with.(using several software packages that dont talk to each other) Isnt it rediculous that we invent a whole industry based on managing websites and content due to this? Ack!!!