It's confusing, but the solution is actualy very, very simple and took me 2 minutes:
If you chose any one of the new tabs Albums, Artists, Genres, or Playlists – then the column browser is not available and therefore greyed out.
If you choose the Song tab, then you can turn on the column browser (View > Column Browser or ⌘B) as well as the sidebar (View > Show Sidebar or ⌥⌘S), and the layout is almost identical to the previous iTunes.
The reason is that while the new layout seems like a downgrade in functionality, in fact the tabs are additions to Songs – which was actually the way the previous iTunes was structures with the column browser. We were always looking at songs, with the column browser and sidebar to navigate genres, albums and artists.
No reason to downgrade or reinstall or panic!
Best,
Thomas