When describing 'ordinary people' I have heard both '凡人' and '普通の人' being used. What is the difference in meaning between these words?
In terms of what they refer to, I guess there are no big differences.
The nuance of 凡人 is negative-neutral. It means banal, ordinary as opposed to special, genius.
The nuance of 普通の人 can be negative, but more often neutral-positive. It means ordinary, normal as opposed to abnormal.
(added) Thus calling someone 凡人 is an insult, but 普通の人 not necessarily so. 普通の人 is usually meant as a reasonable person, but in some context could mean 'a guy just like any other' and be taken negatively.
- アインシュタインの考え方は凡人とは違った. Einstein's idea was different from banal people's.
This sounds praising Einstein's brilliant idea which people cannot think of. Here it is also possible to use 普通の人. It may mean the same, but could also mean Einstein was lacking in common sense.
- 普通の人は麻薬はやらない. Ordinary people don't do drugs.
Here, the speaker thinks negatively of drugs and says normal people don't do it. Using 凡人 here would suggest the speaker thinks doing drugs is a good thing and sounds like "ordinary people don't dare do drugs".