A simple explanation of what makes something defamatory ... does it make people (a) think less of the person it is about, or, (b) make people want to avoid that person.

It's not necessary for the statement to say that the person is bad, but if it says that they are not good at their job or lack qualifications, that is enough.

An example is if someone says "X is a nice person but can't do surgery well" about a surgeon, it would be damaging to their reputation because it affects their profession, but if someone says the exact same words about a person who isn't a surgeon (perhaps, for example, where it turns out the person is actually a plumber), it wouldn't be considered defamatory.