Open and Save dialogs
Windows does not, to my knowledge, provide a reliable and supported way to replace the Open and Save dialogs. (Heck, as already discussed in this thread, Windows doesn't even replace its own dialogs with the newer versions; programs have to explicitly ask for them to avoid breaking compatibility with software which expects and modifies the older versions.)
I know there are programs which do replace the dialogs, presumably via some kind of hack, but the anecdotal stories I've heard from people who have tried using them suggests it results in a lot of problems.
I don't like the Open and Save dialogs either but, given the above, it does not seem like a sensible thing for Opus to attempt to do. Changing the program which gets launched when another program "executes" a folder is very different from changing the API and memory structures that another program uses inside of itself.
Control Panel
Apart from being able to re-use the window for another path, would there be any advantage in having Opus handle Control Panel rather than Explorer? None of the features that Opus adds for general file management seem useful when viewing Control Panel, but maybe I am overlooking something.