I have always wondered why the windows shades have to be raised during take off and landing. I can't think of any safety issues by having a closed window.
I will answer this with a very long answer, since aviation safety is my day to day job.
The window shade opening is a part of a long process to prepare the cabin for sudden (unexpected) emergencies. Why is that? Well, cabin crew have only 90 seconds to evacuate all passengers in case of emergencies. The process of evacuation itself should not take more than 90 seconds regardless of the size of the aircraft or the number of passengers. So to make the evacuation possible in 90 seconds cabin crew and other airlines staff will prepare every flight in advance to make this possible. One example is the emergency exits, only healthy adults are allowed to occupy these seats, they are also required to read the safety card for emergency exits to make them prepared in case of emergencies to help the cabin crew. They are called ABPs (Able-Bodied-Persons). This is just one example.
Anyway window shades opening is a small part of a long preparation process. Passengers are asked to open window shades before take offs and landings because these are considered to be critical times in aviation. Many things can go wrong during these times (most accidents do happen during take offs or landings), so people are asked to open window shades along with other things (seats in upright position, people seated and buckled etc.) to make everything prepared as if the plane were in emergency. In case something goes wrong (God forbid) things will be ready in advance so cabin crew can easily evacuate people in the short period of 90 seconds.
Specific reasons behind the window shade opening include:
These rules might be different from one airline to another, but in general almost all airlines require their crews to make sure of the window shades during take offs and landings. They also add it in the preparation announcements prior to take offs and landings.
The majority of airline evacuations happen as a result of events during takeoff and landing. The reason to open the windows is to see outside. For example, you do not want to open the emergency exit door over the wing if the wing is on fire. Combine the two and one concludes that it is a good idea to open the windows during takeoff and landing.
My source is a flight attendant that I asked this question of a while back.