The current UI experience has been mentioned repeatedly as a prime culprit to understanding Zope. As a result there is no real focus and there is no set terminology to describe the different elements of the interface. The current ZMI is not very user friendly or attractive, especially compared to competing products. The current interface is very rigid, many people would like extra or different functionality, but the current ZMI doesn't provide for easy extensibility or experimentation. The current ZMI has a very scattered approach to its style guide.
Some screens have a lot of help text in an introductory paragraph, some have none.
Some use italics for optional parameters, some don't. Some have a paragraph that just ends with a button. Folderish things have Undo but DTML Documents don't. The current interface is a bottleneck in the scalability of certain objects, like Folders. Confusing error messages.