Operation results should use AdminLTE/Bootstrap css and should show progress if necessary. They also should be stored in session for some use cases, because user want to see error one more time.
Exceptions are caught and handled quite wildly. Task/result is created for every small invocation. Some exceptions are not even logged (mostly in old code). There is no common way how to handle and process exceptions.
Breadcrumbs are not present and Back button is working only on some pages with limited functionality.
We should use main modal window which is currently attached to PageTemplate, there are couple helper methods to work with content and title of this dialog. This way we can get rid of nested form elements which causes troubles when updating wicket models.
AdminLTE template is not compiled from LESS, currently we have precompiled CSS files.
Many different styles and widgets are used for the same purpose. We should use the same widget every time for the same purpose. The entire GUI should have uniform look and feel.
Things that are done frequently should be very easy to do. Ideally one click.
Concrete usability issues: