One feature I wish filesystems had is transaction support, ie:

  begin transaction
    create file X if it doesn't exist, otherwise abort transaction
    write some stuff to file X
    open file Y
    overwrite some data in file Y
    if <insert condition here> commit transaction
  end transaction

Nested transactions would be required as you'd have no idea whether you're already in a transaction.

Something like this can be coded (painfully) with renames, but it's unreliable, hard to test, and hard/impossible to coordinate with higher-level modules.