It looks like the sidebar on the left serves as navigation, so the “new topic” button seems misplaced there. It aligns more closely with the options on the top bar, next to the search and chat button which are also actions. After all, although the a new topic defaults to the current category, you can actually create a topic anywhere on the forum.
As highlighted earlier, I also found the abundance of ways to access categories and tags to be confusing. I am in favor of removing the drop-down menus for categories on tags currently on the top right, since all of that is handled by the sidebar.