That bug has been opened on https://launchpad.net/distros/ubuntu/+source/gnome-session/+bug/32194

"in gnome-session-properties 2.13.91-0ubuntu1, there is no way to set the order that startup program will launch. The list items are not drag/drop capable, nor is there a "move up"/"move down" button combination on the Startup Programs tab. When Adding/Editing a startup program, there is no option to specify a starting order (used to have a number fiedl defaulted to 50 in breezy). On top of this, the order I add startup programs in isn't maintained. If I add mail-notification, compiz, and gnome-window-decorator, the next time I log in they will be listed in this order instead: compiz; mail-notification; gnome-window-decorator. So it's not even resorting them in alphabetical order! But it does re-arrange them into the same order every time.
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> What changed about that dialog? Could you make a screenshot of the dialog you use to change the priority?
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That field is gone. I've attached two screenshots.

http://librarian.launchpad.net/1831300/Screenshot-Edit%20Startup%20Program.png
The Startup Programs tab

http://librarian.launchpad.net/1831301/Screenshot-Sessions.png
I mislabeled that first screenshot. That one was the editable properties of a startup item. This one is the Startup Programs tab of the Sessions window.
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It used to look like this http://www.fs-security.com/docs/pics/sessions-small.png
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I have the same problem on Dapper Flight 5, with the added problem of changes not being saved. More precisely, I can add programs to the startup list (with no priority info), but if I delete one, then close and restart (the command for me is gnome-session-properties), the deleted program remains on the list.
Is this worth a separate bug report?"