The possible (but luckily averted) fate of Evey at the beginning of the novel/movie. Particularly in the film, where the Fingermen are going to rape (and allegedly murder) her for just literally being in the wrong place at the wrong time.
The fate of all those captured by the Fingermen. After they are captured they just disappear. No one (not even their closest relatives) knows what happens to them after they've been taken — apart from the Fingermen themselves. Even the ordinary police don't know it.
Evey's memories of the loss of her parents. Her mother died of starvation when she was a child and the Fingermen arrested her father for an undisclosed reason.
Gordon's sudden death by getting stabbed in the head with a cutlass.
Eric Finch's hallucinations when he takes LSD at Larkhill. He first hallucinates two sacks hanging from the barb wire as the headless, limbless torsos of a man and a woman.
How Norsefire used the FATE supercomputer. Not only is it Big Brother Is Watching You, but they try to portray as being all-knowing, up to the point of able to make up-to-the-minute accurate weather predictions (one of which actually comes to pass). Even Oceania didn't go that far with their Big Brother figurehead.
The single line of "Africa's not there anymore!" after a nuclear war. Africa is the second-most populated continent on Earth, so even the deaths of most of the people of Africa alone would mean a death toll exceeding that of both World Wars put together.
This also raises the question of "why did they nuke Africa?"
The "experiments" Dr Surridge performs on the prisoners involves injecting them with mutagenic chemicals, causing them to either sprout extra appendages or go irrecoverably insane. Like V.
Everyone knows that when the government takes away the elderly to live in nice retirement homes, it's a Deadly Euphemism for gas chambers. Except it's not even that: just some thugs with lead pipes.
In the comic, the fact that V and Evey only really achieve the collapse of the government. The final pages show uncontrolled riots, looting and degradation. At very least Norsefire kept order after the war, now Britain shall not even have that.
That virus. It's been called the most terrible biological attack in history, causing approx. 100,000 (one hundred thousand) deaths in the UK alone. As it's most likely the infection got into the other countries with infected people, one is left to wonder how many lives were claimed by it abroad.