Mostly you should consider your local and state regulations. I'm assuming your talking about car windows.
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I agree that first and foremost you should consider any applicable laws.

Beyond that though you might also want to consider how good your vision is, whether you do a lot of driving at night, and whether you keep valuables in your car.

If you live in an area that is sunny all the time and you drive mostly during the day then heavily tinted windows isn't such a bad thing. If you drive a lot at night the tinted windows will make it extremely difficult to see. If you have poor vision, factor that in as well.

Nobody plans on it, but theft is always something to think about. If you leave valuables in your back seat you may want darker windows. If you leave children in your back seat (nobody plans on leaving them in a car for long, but if somehow they are in distress and can't get out dark windows will prevent bystanders from seeing them) you might want lighter windows.
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Here a few things to consider when tinting your windows:

1) What is the maximum allowable tint in your state?  You want to ensure that your tint meets your state's legal requirements, otherwise you can receive a ticket.  I have heard of officers intentionally scratching an illegal tint with a blade during a pullover.  Also note that this percentage can can vary for the windshield. 

2) Do you mind attracting unwanted attention?  Just like driving a bright red or yellow sports car, having an aftermarket exhaust or lowering your car, tinting your windows may attract unwanted attention from highway patrol.

3) Do you intend on eventually selling the car?  As soon as you modify a car, you are lowering the value of that car to any potential buyers.  Why?  Because most people want a 'stock' automobile the way it was originally delivered by the automaker.  As soon as you limit the pool of potential buyers, the asking price has to come down.  Make sure you consider if you would like to sell the car down the road -- and ask the installer about the cost of removing the tint when and if you need to sell the car down the road.
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