Drilling down to see leaves and then drilling back up to the root to see the next branch sounds tedious...unless I can do it the same exact way that it can be done now with a mouse and a normal menu (i.e. without clicking, just hovering).
However, "hover" isn't a touch thing, so I bet that Ubuntu will make that functionality a 2nd class citizen or just cut it out altogether in favor of more a more tedious "touch oriented" (read: clicky) drill-down behavior. Which will suck (IMO).
Aren't drop-down menus technically the same as "drill-down" in terms of number of clicks? As long as you can use "breadcrumbs" that allow you to quickly access the hierarchy of menu items, it shouldn't require any additional clicks to get to the option you want.