to be honest I could not believe my eyes: it took me some hours to find out that WordPress just can not provide a collapsible category navigation. In other words 'show subcategories only for the selected top level category' and so on.
This is such a basic feature that in the beginning I wasted plenty of time searching the documentation thinking 'my fault, I just can not find it' - but in the end I found out it is really not there.
In addition please see this discussion http://wordpress.org/support/topic/223376 - even there seem to be plugins providing collapsible category navigation - the be place to solve such a functionality would be the built in wp_list_categories function, such a method is pretty basic and close to the core.
Parameter suggestion: collapsible, values: 0|1
Where can I open an offical feature request?
Thanks for sharing your thoughts,
P.S.: by expressing my surprise about this topic I don't want to offend anybody, it's just that most other systems have this behaviour by default and with the long development history of wordpress I would not have expected it to be missing.