Window treatments serve three purposes: controlling light, providing privacy, and contributing to the room aesthetic. In that order of priority.
Light Control First
The first question for any window is: how much light do I want, at what times? A home theater needs near-complete blackout. A bedroom needs the ability to darken substantially. Define your light-control requirements before choosing treatments.
Matching Treatment to Window Type
Tall, narrow windows do not work well with curtains that tie back to the sides — inside-mount cellular shades work better. Floor-to-ceiling windows need treatments that stack compactly when open or are ceiling-mounted so they can close fully.