I disagree; on most Linux UIs I've seen, screen real estate is at a premium because of over-generous borders, oversized icons, oversized buttons, oversized listbox item sizes, all generally looking like a 96dpi UI rendered on a 75dpi device (I've often wondered if X Window historical baggage and ancient bitmap font sizes had something to do with it). Everything is about 33% oversized.
(I'm a fan of the relative information density on Windows. It's really noticeable in file explorer views in details mode; Windows Explorer packs in a lot of information with very thin borders so the text on each line stays large (though the icon for the file is only 16x16), while the nearest equivalent on pretty much any Linux shell file explorer I've used has large inter-item borders that force the detail text to be tiny; and it still doesn't get in as many items as Windows.)
At least in GNOME and XFCE you can decrease the default font size, which is what I did right after installing Debian with XFCE. XFCE also has really slim buttons and borders. Sadly the file manager, Thunar, is not slim at all.
So if you have not already given XFCE a try I recommend you do. Personally I wont use Unity until the skin i as slim as XFCE.
(I'm a fan of the relative information density on Windows. It's really noticeable in file explorer views in details mode; Windows Explorer packs in a lot of information with very thin borders so the text on each line stays large (though the icon for the file is only 16x16), while the nearest equivalent on pretty much any Linux shell file explorer I've used has large inter-item borders that force the detail text to be tiny; and it still doesn't get in as many items as Windows.)