Torque is one of the most critical aspects of spark plug installation. Torque directly affects the spark plugs' ability to transfer heat out of the combustion chamber. A spark plug that is under-torqued will not be fully seated on the cylinder head, hence heat transfer will be slowed. This will tend to elevate combustion chamber temperatures to unsafe levels, and pre-ignition and detonation will usually follow. Serious engine damage is not far behind.
An over-torqed spark plug can suffer from severe stress to the Metal Shell which in turn can distort the spark plug's inner gas seals or even cause a hairline fracture to the spark plug's insulator...in either case, heat transfer can again be slowed and the above mentioned conditions can occur.
The spark plug holes must always be cleaned prior to installation, otherwise you may be torquing against dirt or debris and the spark plug may actually end up under-torqued, even though your torque wrench says otherwise. Of course, you should only install spark plugs in a cool engine, because metal expands when its hot and installation may prove difficult.
I believe that slowr8 is incorrect in saying to tighten half a turn once finger tight. That might be OK for spark plugs with gaskets but not for plugs without gaskets. Should turn only 1/16 of a turn once finger tight, or use a torque wrench - 1.5-2.5 kg/m for cast iron cylinder heads and 1.0-2.0 for aluminium heads (14mm spark plugs). Also use nickel anti-sieze on the threads when you replace the plugs to make them easier to undo next time.