[AhtiK]

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.

[krakensden]

Unity and Gnome have that disabled by default :(

[dethstar]

in GNOME you can do it by pressing the windows(super) key.