Carbon is a KDE4 plasma theme I did some years ago but since then I've learned a lot about SVG. Plasma themes like the default Air theme use bitmaps for the shadows(some would call this cheating a bit I know), but Carbon will be fully vector. This gives a more crisp, clean look to the theme.
Check this screenshot out of the vector shadows [link]
Carbon actually works from KDE4.2 to the current version KDE4.8, which is great compatibility. Although Plasma themes are more work than GNOME-Shell themes, due to being purely image based rather than CSS, it does have it's advantages and disadvantage and requites advanced knowledge of SVG image editing with apps like Inkscape.