In addition to what AndrewD said: Some cheaper models of refrigerators and kegerators use a much less sophisticated technology to operate -- they just have a cold plate which chills the air inside the typically smaller fridge box.
I am no expert, but it appears that many older, and often many efficiency size fridges use this same concept by fabricating the entire ice box out of a cold plate and allowing the ice box to cool the air inside the fridge. These would be models which require de-icing every now than then, because there is no auto-defrost. The more ice on your cold plate, the less efficient it will cool the air inside the fridge. Down side to these is the speed at which the box cools -- much slower with a plate than with a fan.
I would agree that any time you have a fridge with an external coil loop, you should be extra careful to enhance ventilation if you install this in tight quarters. I installed a freestanding Haier kegerator under my counter (the instructions say not to do this) and cut in a relief louver in the back (no fan) to allow for some air circulation, just to avoid the head from building up and having no where to escape. I haven't had any issues to date with cooling capacity -- just with ice build up, but it's within normal limits.
Generally speaking, really nice, expensive kegerators (True, Beverage Air, etc) are fan cooled units, whereas cheap kegerators (Haier, Danby) are plate cooled. If I had the money I'd get a nice one, but given that I'd rather spend the money on the things I can see and touch -- well the balance for me shifted to spending money on the tower and taps.
Also, never use a sharp object to remove ice from your coils or chiller plate -- one wrong move and you could permanently damage the coils. Either use a blow dryer or allow for it to melt on its own.