Look how beautifully these drapery panels allow the full width of glass to show. Who would want to cover one more inch of these beautiful windows and lovely view than they had to? These draperies are perfectly placed and sized.
This avoids Mistake No. 1: Poorly placed or sized window treatments.
The rest of this list may be in no particular order, but I admit that this mistake is my number-one pet peeve and probably the mistake I do see most often.
Drapery rods end just at the edge of the window so that when the panels are pushed all the way open, they still cover quite a bit of the glass. If you want to keep as much light as possible, use a rod that extends past the sides of your window by the full width that the panels take up when pushed all the way open. That width is called the "stack."
How many inches wider than the window your rods need to be will depend on how wide the panels are, what kind of fabric is used, and the type of attachment to the rod — grommets, metal or wooden rings, tab tops and so on.