As you've already mentioned, light and privacy needs are two of most important considerations. Many people like windows on the front of the house to have the same treatment for uniformity from the street view. That could mean blinds or shades in every room (with the same lining, usually white, if using fabric), or drapes (again, with the same lining). If your house has an asymmetrical facade, then it might not matter so much.

Once I've addressed light and privacy concerns, then I think about what look or feel I want in the room. Do you had a specific style or taste? Certain furnishings you need to work with? Personally, I've done bamboo shades with side panels and padded cornices in the office and living room, full drapes with board-mounted valances to cover the hardware in the MBR, ring top panels in the family and dining room, tab top valances in kitchen and laundry, wood blinds with fabric valances in bathrooms, and pole pocket panels most everywhere else. Some might want more continuity if the floor plan is very open. Since I sew and love fabric, I make whatever I want for each room, making sure materials and styles flow but not always using the exact same treatment.

Other considerations might be allergies (wood blinds with no fabric treatments would be best), sound absorption (in large, echo-y rooms, drapes can be a great dampener of sound), and cleanliness (long panels or lots of fabric in kitchens or bathrooms is usually a lot of upkeep).