it's a good question - I avoided it in my own response with the clever use of quotation marks.

But to answer, I think the term is used colloquially all of the time and of course is open to interpretation.

I would suggest it has nothing to do with a drug's pharmacology or chemical structure but rather the degree to which a drug when taken in easily-consumed quantities can shape our perceptions of the world, the likelihood of negative externalities due to consumer behavior and the probability of becoming addicted to the drug.

A mixture of those things makes a drug "hard" in conversational language e.g. something that dramatically changes a persons perceptions, frequently has negative externalities and can cause addiction with short-term sustained use is a "hard drug". Like alcohol.

When addicted to such a drug, the negative externalities typically expand in scope and severity and if the use scales to a significant portion of the population would generally be regarded as an undesirable state for society to be in.