1. Unity vs. Modern UI
Both Microsoft and Canonical have received considerable flak for the default user interfaces in their respective OSs. In Microsoft's case, of course, it's the Modern UI, formerly known as Metro; in Canonical's case, it's Unity. Both are designed with touchscreens in mind, and borrow heavily from the mobile world.
By removing the Start button and overhauling the way users interact with the operating system, Windows 8's Modern interface poses a considerable challenge for users, who face a significant learning curve.
Unity, on the other hand, became a default part of Ubuntu back in April 2011 with Ubuntu 11.04 Natty Narwhal. It has definitely undergone growing pains, but more than a year has passed, and Canonical has revised the interface accordingly. Although it still has numerous critics, most people concede that it has matured and improved. Some observers, in fact, have even suggested that it may feel more familiar to many longtime Windows users than does Windows 8.
Linux has long been known for its virtually limitless customizability, but given the current controversy surrounding desktop interfaces, that feature has become more salient than ever.
This is a point on which Windows 8 and Ubuntu differ considerably. Yes, Windows 8 does allow users to customize some aspects of their environment, such as by specifying the size of Live Tile icons, moving commonly used tiles to the left side of the screen, or grouping tiles by program type.
Most of the changes you can make in Windows 8, however, are largely cosmetic, and they don't include a built-in way to set the OS to boot to the traditional Windows desktop. A growing assortment of third-party utilities such as Pokki can restore that capability, but otherwise you're stuck with Modern UI. Windows 8 offers what you might call a "tightly coupled" interfacein other words, one that you can't change substantially.
Ubuntu's Unity, in contrast, is more of a loosely coupled UI. First and foremost, you can easily replace it with any one of several free alternatives, including KDE, Xfce, LXDE, Gnome 3 Shell, Cinnamon, and MATE.