Aqua windows are essentially borderless. These effects frame the window enough to make it distinct while still allowing items behind to show through. An extremely minimalist window border treatment is therefore possible. More of each window may be dedicated to displaying content, rather than UI widgetry. Advanced users sometimes want this functionality to allow them to work more efficiently in certain window manipulation situations.
Sheets are a good idea.
Sheets keep window-specific dialogs attached to their window. Dragging the window around also moves the attached sheet. The use of sheets helps avoid application-modal dialogs, letting the user continue to use an application while deferring decisions about a particular window in that application indefinitely. The attached nature of sheets reminds the user which window goes with which dialog. When a document has unsaved changes, a small dot appears inside the document window's close widget.