Currently nautilus opens the home directory by default.
However the command
nautilus /path/to/a/directory opens nautilus with the selected directory open.
How do I edit the unity bar Home folder icon to include path to the directory of my choice?
Boot system in classic
If you are opening nautilus from Applications->System Tools->File Browser just right click on the Applications menu select "Edit Menus" select System Tools in the configuration window then right click on File Browser select properties remove the %U from the command line and replace with a new start location ie: / ( then close all the windows (this sounds more complicated than it is sorry) if you are starting nautilus from some other launcher just do the last bit.
May be this will help
Have a look at the answer to How to add my favorite places as a quicklist in my home's icon in Unity?
You can change the first Exec line to read
Exec=nautilus Documents and it will open your Documents folder.