Many of the Doctors enemies are scary, but the Cybermen, and also the Daleks are enemies who keep reappearing and never lose their scare factor.
The Dr Who Cybermen strength
The Cybermen are also known as men of steel. Their metal frame makes them significantly stronger than the humans which they were made from, probably the strength of ten men. The Cybermen have hands like humans and are very dexterous and hence able to utilize their strength effectively. The tough metal exo skeleton also helps to protect them so they can face their enemies when attacking without needing cover or protection.
The Dr Who Cybermen emotions
The Cybermen have had all their emotions removed. They are therefore unable to comprehend the effect that they are having on people or to empathize in any way with others beings. This lack of emotions also means that Cybermen cannot feel pain, not only are the unable to understand suffering, but they cannot suffer. These robotic bipeds are either in fully functioning order or not, there is no in-between.
The power of the cyber controller
Since the Cybermen have no emotions or pain all their decisions are based either on facts, or they are following orders. The Cybermen are capable of analyzing facts and then deciding on their action, but this information needs to be fed to them and hence they need to have a master controller. The cyber controller will decide on orders and the actions Cybermen take when they have analyzed data. For example it will be the cyber controller who will decide either on request or through preprogramming if someone will fit the criteria to be converted or deleted. The cyber controller is all powerful.
How to stop a cyber world
A brief skim through modern history can easily show the effects of a dictator, the successes and pitfalls have been well analyzed. The difference with the cyber world would be the lack of emotions. Every dictator has had opposition either openly or as a part of an underground movement. But this opposition is driven by a desire for change, usually to make a place happier and to reduce its misery. The determination of the opposition is fuelled by the suffering around them. Without the emotions of happiness and sadness why
would anyone stand up to a dictator? What would be the purpose of change if you were never aware of being happy or sad? Equally what would be the purpose of progress if you had no emotions? For progress to happen, sacrifices are usually made, and a sacrifice is a risk based on an evaluation of both the associated emotions and the possible outcomes.
So, perhaps the reason the Cybermen are such a successful creation is that they represent our world with the removal of everything which is important to us. But most importantly, it would be a world without hope. If the Cybermen were successful then there would be no one left to oppose them and their actions would become irreversible. The Cybermen scare us not just because of their strength and ability to delete, but because they are the cog in a system which terrifies us.