I've finally got around to starting using EAP builds of Demetra, and have some initial feedback, and here it's to do with cosmetic and usability issues.
First of all, whilst the Alloy look and feel gave a more polished option to the user interface as of IDEA 5 (although I imagine Mac users would still prefer the native integration, especially with the quaqua look and feel tweaks...), things like the menu bar and toolbar of IDEA do look a bit, well, dull really. Dated. Whatever. That's in comparison to other IDEs (not just Java ones) and other desktop applications in general. I've noticed for example that in the forums, you have (for some time now) been using the IconExperience icons: I also bought them for use in personal projects, and find that other applications (e.g.: DbVisualizer) benefit from them by being more modern and polished. Some of the more specific "IDEA" icons, in say the "Project Settings" dialog are very specific and very clear, so I've no problem with them, however the "standard" file/undo/cut/copy/paste/etc icons are a bit dated really and it's worth at least trying to see what the iconexperience ones look like (I assume you already have the licence as I see them here on the forums).
I'm also a user of the JIDE products, and think that for example their "action framework" would certainly add polish to your menus and toolbars. As you already have a perfectly functional implementation of docking-type panes, I'm not suggesting you adopt their docking solution however.
Going further than that however, I've also been trying out the Microsoft Office 2007 beta, and in particular the "ribbon" UI component which replaces the toolbar. It's something that takes a bit of time to get used to, but in fact I think it could really be useful in the case of IDEA. For example, a ribbon for basic operations, another for refactoring operations, another for debugging, another for source control, etc... When you master IDEA, it's certainly much faster via the keyboard, but there's so much hidden stuff that that could in fact be an excellent solution to expose the full set of features in a task-oriented way.
Hope that's useful feedback for you.