Why open up the window shades before takeoff and landing?
I'm not sure about other countries, but in the USA there is no FAA requirement mandating that the window shades be open for takeoff and landing.

Individual airlines vary in their procedures, with some requiring them to be open while others do not.  This is set by company policy, presumably set by management because they decided that there was/was not a safety benefit in doing so.

As a pilot, I would prefer the window shades to be open all of the time so that even passengers have the chance to point out something that they think might be a problem.  For example, recently a passenger on an airliner noticed a fuel leak and told a flight attendant about it before the airplane took off.

One important thing to keep in mind is that in the event of an emergency evacuation, you should always look out the window before opening an emergency exit to make sure that there isn't a fire or other hazard just outside the door.  This is why a lot of emergency exits don't even have window shades on their windows.

This answer on Skeptics.SE also points out that emergency crews responding to a crash can see through the open windows to better respond and help the people on the airplane.  It also mentions that the passengers eyes will also be more acclimated to the lighting outside the airplane if the window shades are open.  This is also why some airlines have policies requiring the cabin lighting to match the outside light (i.e. the lights will be on bright during the day and dim/out at night).

This answer on Travel.SE also goes into more detail from a person who works in the airline safety industry.