I second this idea! (I was going to suggest it myself but made sure to search first and found this thread).
With the new Seasons expansion coming out, it would make even more sense. Say its a blisteringly hot day and your Sims want to stay cooler-- they could close the curtains to let in less sunlight/heat. Or if your Sims want more privacy it would be nice to go from closed to open.
Blinds and shades that open and close would also be a nice idea.
It would also be good for vampire Sims. They could close their curtains during the day and open them at night.
There could even be different types of curtains
*Blackout curtains that completely block the light when they are closed
*Regular curtains that maybe allow in about 20 to 30% of the light when closed
*Translucent / Diaphanous curtains that allow about 80 to 90% of the light in but afford some privacy.
Perhaps there could even be a shade or center curtain that lets in more light but obscures view from prying eyes.
I almost forgot to mention there could be shutters on the outside that can open and close. So, say there is a big storm expected (again with Seasons) and Sims want to close their shutters to protect the windows a bit more.
Another suggestion is something that would be listed as a curtain but it would be more of a film/overlay on windows. It would allow for frosted, bubbled, & textured windows for bathrooms & more private spaces.