ZK is missing a simple mechanism to trigger plain notifications for instance on successful CRUD operations.
The Messagebox.show() approach is a good starting point for this component.
However, it always requires user interaction thus hindering the workflow too much to be used as a mere visual feedback.
In addition to requiring such feedback, e.g. having to "click away" an error message, notifications should optionally just disappear, or better yet fade out, either based on a timer and/or mouse movement.

The Vaadin framework has a simple, yet flexible enough implementation for such notifications, that would be great to have in ZK.
http://demo.vaadin.com/sampler#Notifications