Categories

Tags could be used as cumbersome workaround. But without editing options they are not a replacement.
Reading Dave's blog post, looking at his screenshots and all the new features regarding the sidebar (including collections) it's quite clear that you shifted the focus from individual users to enterprise users.
All explanations I've seen so far emphasize how good the change fits to users with lots of accounts and vaults. For an ordinary user with just 1-3 vaults and one account (or even a family account) the new sidebar is pretty much a waste of space.

Why not let the users decide if they prefer categories in the sidebar or vaults/accounts? Yes, I know every config option means additional testing required. But some parts of a UI simply doesn't fit every use case.