Mozilla Thunderbird 5.0
I appreciate the update, but the Thunderbird 5 UI on Windows 7 looks like a dog's breakfast and is possibly the worst implementation of Aero I've ever seen.
Despite my darkish color setup, the toolbar buttons are difficult to read, have jagged corners, and are just plain ugly. Their glow effect on hover is too slow to even go noticed, nevermind that it's inconsistent with the rest of the UI - no glowing effects elsewhere.
The translucent glass is used as a primary background for the toolbars region of the window, leaving the elements to appear as though they're just floating around and don't belong to the rest of the UI. It looks and feels incomplete, like a layer is missing.
When customizing the toolbar, there's a checkbox option for "Use Small Icons". It does nothing.
So much ugly, UI team. But then again, it's a brilliant client. And free. ;)