Doctor Who: 5 Reasons the First Cybermen Were the Scariest
Except for the first Cybermen. They do suggest taking humans back for conversion into other Cybermen, but that that is completely incidental. It's just an afterthought. Their real goal is simply to siphon off the energy of the Earth to restore their own supplies, and if it causes the death of everyone else, then so be it. In this incarnation alone do the Cybermen combine all their most terrifying aspects with those of creatures like Daleks and other enemies. In a sense, this is them at their most deadly if not most powerful.
The Realism: The modern Cybermen seem utterly unrealistic. Even the latest redesign for Neil Gaiman's "Nightmare in Silver" is still too cartoonish to bring real fear. They might as well be the cybernetic ninjas from the first season of Mortal Kombat: Legacy. Well done, but ultimately they could be CGI and you would hardly notice, especially with that ridiculous lockstep marching and the catchprase "Delete."
If something like Cyberconversion actually did occur, it would likely look much more like the first Cybermen. Sure, I'll stipulate that the modern ones represent the result of a high-tech industrialized world unlike the world of 50 years ago, but that actually makes it even worse. How can you be frightened of what is essentially a murderous, mobile smartphone app, operating in perfect sync with its fellows? No, the last vestiges of individuality and the ugliness of the meshing of man and machine are much more frightening than the animated suits of armor we have now because it's really not all that far off in terms of possibility.
The Hidden Message: "The Tenth Planet" was William Hartnell's last outing as The Doctor. Poor health drove him from the role he had grown to love. Human frailty finally turned a hero to millions into just another man.
And what does in the First Doctor narrative-wise? Old age and weakness from one battle too many and the grand, energy-sucking demands of a coming new world he is not fit to face in battle. The Cybermen come to Earth as a possible future as well as a threat, and the man that has already become our planet's protector is almost powerless to stop it. In the end, doing so kills him.
Maybe that's what scares me the most about those clunky, plastic first Cybermen. They are undone by the very flesh they claim to have overcome, but they take even The Doctor down with them. It's a huge metaphor for the struggle of life over weakness, and the ambiguity of it cuts deep.