It was roped off in 8.5 due to some problems that were not able to be fixed by ship time. It will be back in 8.5.1.
In 8.5, you can choose to have all documents (not applications, unfortunately) open in a new window - from Preferences, choose Windows and Themes, then choose Window Management option of "Open each document in its own window".
Note, a lot of people like the new Window Management option "Group documents from each application on a tab" - you may want to try that out.
Feedback response number JPKR7S7PGK created by Jenifer P. Kidder on 05/19/2009