You can currently (it’s day one so in day two it might change!) go to the forums menu item and click recent posts. That will show all the posts not filtered by category. If you want to respond though, it will take you into that specific forum.
There will be formatting things which will be cool, and some will suck. Many, including me, are used to the UFN formatting but in order to have “categories” they have to split. That doesn’t mean down the road I might be able to work some PHP wizardry to create a threaded, all-topic layout though.
This is just DAY ONE! Just imagine.