This is a summary of differences between the Metal
L&F specification for the new window system and
the current implementation. All issues should be
fixed:

Document tabs:

1) the backround color of selected document tab
(not activated) should be   [230,230,230].
Currently it is [234,234,234].

2) the drag texture in document tab is not the
same as in the spec

3) the tab border is not precisely the same as in
the spec

4) the partially visible tabs do not look the same
as in the spec

5) the scroll buttons in document tabs do not
follow the spec

6) the selection background color doesn't fill the
whole tabs width (when the leftmost tab is only
partially visible)

7) the dividing lines between tabs and document
contents are one pixel shorter

Window title bars/tabs:

8) missing drag texture

9) the close button doesn't correspond to the
button from the spec

10) one pixel is missing from the bottom border of
window title bar

Window area:

11) the window area should have one pixel empty
border at the top and left side

12) the splits should be only 4 pixels thick,
currently they are 6 pixels thick 

http://ui.netbeans.org/nonav/docs/ui/ws_issues/ws_issues-metal.html