I think I understand when to use metal or aqua. Tell me if you agree.
If the app separates the concept of a window from the concept of a document, use metal.
If window == document, use aqua.
If both happen in one app (eg mail, viewing vs composing) then pick one at random
Think about it. The finder in Panther, when in metal mode, has 'browser' windows. In aqua mode then one window is equivalent to the folder it represents.
In iPhoto, then the window isn't tied to its content. Close the window and you don't lose anything. Ditto itunes. iMovie has one window holding lots of clips, etc, so I can see why it's brushed.
Safari also doesn't have one window == one document. For a browser, that would be really silly - every link would open in a new window, unless you already had that page open... in which case it'd pop to the front.
Mail's aqua because you can do one window per document (if you hide the preview pane)... and composing messages always works this way.
Quicktime Player I think is wrong. It should be aqua using this scheme.
Amorya