The relationships are very clear in most code using Qt. When you create a QObject-derived object, one of the constructor parameters is always the parent. Little things like this goes a long way. I never felt the need to express ownership more explicitly with Qt. Whenever I felt the urge to do so, it turned out that I was trying to do something crazy that I probably shouldn't try in the first place...
I think my point though is still valid: that it's implicit ownership rules which are not clear ownership rules.