I'm struggling also with the missing categories! It was more than time to give the forum page a new face, but the lack of all categories as we had it in the old forum, is just not practical anymore.
I found the categories in the old forum very helpful. As mhe mentioned, it makes sense to browse a forum to see what problems others are having before upgrading to a firmware, but then you don't have anything to search for and so I find the presentation as it is now insufficient. There are not only cases where the forum provided helpful articles for acute problems, but also where you could just read through topics to see what other users are having problems with e.g. after updating to 7.0.3 or whatever. Please add this view option again.