If you change the default view from “Latest” to “Categories”, the place where you’d expect the “Create Topic” button to be is currently empty for regular users. For admins, that place is taken up by the “Create Category” button. If it is possible to create a new topic from the front page when it is configured as “Latest”, the same should be the case when you have “Categories” as the main view.
I do realize that the “Create Topic” button appears when you go from the categories view to an actual category, but expecting users to do this feels like a minor UX inconsistency. In one case, where “Latest” is the main view, users can create a new thread immediately from the front page, and choose the category in the composer. In the other case, where “Categories” is the main view, they can’t do this – they have to go through what seems like a sub-optimal step – clicking through to a category and then hitting “Create Topic”, rendering the composer’s – in my opinion – preferable way of selecting the category “un-utilized”.
Edit: Hm. Ten minutes later: I’m not sure if the above really constitutes a problem.