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Slowing down the shutter speed will make the image look much more blurred and choppy, but it will also lighten it a great deal.
This often makes lowering shutter speed a last resort option for situations that are too bright.
It can also be an interesting way to add a video effect or give everything a blurred feel as action occurs.
This can be fine, though it is often better to do this in post-production than permanently alter the image with shutter speed.
If you need to lighten the image you are also going to want to open up the iris more before trying to slow down shutter speeds.
Image: http://www.flickr.com/photos/chethan2u/3758866817/