Low sills (meaning the bottom of the window is closer to the floor line) create windows that tend to have the proportions of doors. As such, they provide stronger connections between indoors and outdoors. Using lower sills also can make a smaller room feel much larger.
Tall heads (meaning the top of the window is closer to the ceiling line) provide much the same visual impact as lower sills. In fact, a conservatory derives much of its visual impact from the lack of any substantial wall structure above the windows.
Deep reveals (shown in the first photo below) enliven a window by providing not only a place to put things but also by emphasizing the three-dimensional quality of the surrounding wall. The deep reveal gives us a sense of the weight and solidity of the wall and the security the wall provides.