The ability to change subdomains should be less prominently displayed.
In the new layout, the "Boardgames" and "RPGs" panes are to the left and just above the bar for the dropdowns most people will use. Hovering over "Boardgames" gives a dropdown of "subdomain" filters. "All" lets you see everything while "Wargames", for example, lets you see only wargame related things.
The subdomain feature may well be great for some BGG users. I know it has been requested. However, most users will never access this feature. Most who do use it will apply a subdomain rarely, and many will do this exactly once. For example, many who are only interested in wargames will set their subdomain to "Wargames", and then never change it.
Moving your mouse slowly from the upper corner towards "Home", "Browse" or "Forums" will cause the subdomains dropdown to fall, thus obscuring those other options. This can be annoying and those other options will be accessed much more frequently.
One possibility is to turn off the "Boardgames" dropdown and just have an option to change subdomains, elsewhere. Another possibility is to put the "Boardgames" and "RPGs" panes right justified on the same line as present. There are probably better options that haven't occurred to me.
My basic suggestion is that rarely accessed features not interfere with commonly accessed features.
I think overall the UI change is an improvement.