Weapons of Mass Destruction. Nuclear, chemical and biological weapons. They're really nasty, right?
Well, some of them are. Nuclear weapons are Very Destructive Indeed. Even a tiny one, detonated in the middle of a major city, would probably kill hundreds of thousands. A medium-sized nuke could kill millions. The biggest would wipe a small country off the map in one go.
Chemical and biological weapons, on the other hand, while hardly nice, are just not on the same scale.
Sure, there are nightmare scenarios - a genetically engineered supervirus that kills a billion people - but they're hypothetical. If someone does design such a virus, then we can worry. As it is, biological weapons have never proven very useful. The 2001 US anthrax letters killed 5 people. Jared Loughner killed 6 with a gun he bought from a chain store.
Chemical weapons are little better. They were used heavily in WW1 and the Iran-Iraq War against military targets and killed many but never achieved a decisive victory, and the vast majority of deaths in these wars were caused by plain old bullets and bombs. Iraq's use of chemical weapons against Kurds in Halabja killed perhaps 5,000 - but this was a full-scale assault by an advanced air force, lasting several hours, on a defenceless population.
When a state-of-the-art nerve agent was used in the Tokyo subway attack, after much preparation by the cult responsible, who had professional chemists and advanced labs, 13 people died. In London on the 7th July 2005, terrorists killed 52 people with explosives made from haircare products.
Nuclear weapons aside, the best way to cause mass destruction is just to make an explosion, the bigger the better; yet conventional explosives, no matter how big, are not "WMDs", while chemical and biological weapons are.
So it seems to me that the term and the concept of "WMDs" is fundamentally unhelpful. It lumps together the apocalyptically powerful with the much less destructive. If you have to discuss everything except guns and explosives in one category, terms like "Unconventional weapons" are better as they avoid the misleading implication that all of these weapons are very, and equivalently, deadly; but grouping them together at all is risky.
That's WMDs. But there are plenty of other unhelpful concepts out there, some of which I've discussed previously. Take the concept of "major depressive disorder", for example. At least as the term is currently used, it lumps together extremely serious cases requiring hospitalization with mild "symptoms" which 40% of people experience by age 32.