I first saw "Threads" in high school, and saw it again recently as a grown adult. It does make a measure how old you are when viewing this movie; you take the actions on screen more seriously.
"Threads" plays like BBC documentary about a catastrophic nuclear war, interjecting live scenes with a bland monologue and various statistics, although one wonders what audience would be viewing this documentary.
Since it does play like a documentary, it feels no need to either overplay events or sugarcoat things for our sensibilities. There's no speeches or heroic actions, everything occurs as it happens, no matter how horrifying.
The gore is moderate (it was a TV movie after all) but is unsettling because it's taken to be real. Throughout you look for some hopeful thought to intrude, even comic relief, but "Threads" stares you down, making you watch the horror and woe to the bitter end. There is no hope or salvation, only despair.
It's worth seeing a movie like this as a reminder of the horrors of nuclear war; the threat of a mututal destruction by superpowers seems to be fast fading, but there's always the possibility of terrorists or new enemies.
"Threads" is to nuclear war what "Saving Private Ryan" is to war movies, a landmark film that delivers a strong political message without ever really mentioning it.