Not sure about Unity but in quite a few Linux desktops you can press Alt and move window by clicking anywhere in the region. That's how I've always moved windows and been surprised when Windows or OS X have behaved differently.

But the question is great! For a regular user dragging window without setting off a menu can become a serious usability issue. Yes, menu won't take the whole bar but still the size of the movable area becomes substantially smaller.

One solution to detect dragging could be to use mouse release event for menu, instead of press and filter out shaky hand who actually wanted menu, not drag. But then again it's still bad because user expects menu system to react ASAP/mouse press, not release.

Relevant image: http://i.imgur.com/Ppp12vB.png

GNOME leaves a small portion of the top bar free for dragging purposes (about an inch on my 17'' monitor)... more than ample space to suit anyone's dragging needs.

Watch the video in the article. At 0:48, they drag the titlebar as usual, even though there are menu entries directly under the mouse cursor. I guess they have replaced the "pull down" mouse gesture with initiating a window drag, so the only way to open a menu with the mouse is to click it.

I think the assumption is that the menu won't take up the entire bar

Holding the button down for longer than some threshold or moving the mouse more than some distance starts a drag.

Hold alt. Drag anywhere. In general, these kinds of questions are best answered by holding down the windows-button until the help shows up.