Basically Drupal is bit complicated for normal clients. Drupal 7 is better but still its not that user friendly for clients especially when comes to admin content or taxonomy!
A good admin theme can help, but it is really just lipstick on the pig.
I will also agree, to an extent, that Workbench can be of use for content creators on sites that are adding lots of new content.
However, the administration problem for Drupal is solved by doing three things:
Only install the modules that you actually need. Once you have a site ready for deployment, disable and potentially uninstall anything that you don't actively need. This reduces the number of entries in the admin menu.
Set up a proper set of roles and configure the bare minimum permissions that each role needs. Even if a single person is running the site, set up a non-administration role for their user that they will need for daily tasks. Save the uid 1 account for occasional tasks. Again, this reduces the number of entries in the admin menu.
Set up as much contextual help as is feasible and train the client. It is especially helpful if you can match up features in your site to a good book, like The Definitive Guide to Drupal 7.
Although Workbench is good I use to build a custom admin with:
Also is good to have menu per role to choose what menus can edit editors.
With this you must built it, but I think you will get a better options that Workbench.
Also you can use dashboard to help user to see their options.
Workbench provides overall improvements for managing content that Drupal does not provide out of the box.
Workbench is probably a good idea for general administration/workflow improvement. But if you're looking to improve UI on a fine-grained level you could set a custom tpl page for adding/editing a node if you don't like the default add/edit view (I know I don't). Or, along the same lines, you could run hook_form_FORM_ID_alter(). I know you're asking for a module or a theme, but thought I'd mention this anyway.