Long exposures can increase the 'noise' of an image. This begins to break up the image and destroys detail. All recent EOS cameras have a Long Exposure Noise Reduction feature that can be switched on in the camera settings menu.
Leonardo da Vinci, © Alessandro Sciascia 2010, Canon EOS 20D
The camera will now remove much of the noise as it writes the image to the memory card. However, this takes the camera as long to do as the actual exposure, so a two minute exposure will take another two minutes before displaying your image on the review screen. You will not be able to take another shot during this time.
Shooting in raw is recommended for night photography as it allows for more adjustment of the image when using Canon's Digital Photo Professional (DPP) software than is possible with JPEG quality files. It also allows you to leave the Long Exposure Noise Reduction switched off and to apply noise reduction in DPP.