I don't think a UI is needed for assigning theme variables and I don't think drupal_render($node['details']) is that much bad for themers, it has an obvious name and function and I don't see why it would be hard to describe it to themers.
<br><br>The issue I see here (which you all are aware of I know, and the UI solution is also trying to solve), and that always bugged me when trying to explain it to themers is that how you know that $node['details'] is available and has what you are looking for. I think a good solution is to have a generated custom help page where it lists all the defined variables and their descriptions. And by letting modules specify a #description attribute when adding to the $node->content we can use that to generate such a help page. I'm not up to date with the changes of the theming layer in D6 so I'm not sure of the do-ability of this.
<br><br>So to conclude, I think drupal_render() is great, a help page would accomplish one of the UI goals but would spare us the trouble of naming variables because as a themer, why would I care if the variable is 'field_details' or 'mydetails'... etc. I will just use it whatever it's.
<br><br>- Amr<br>