The definition of concern can be found in any dictionaries like this link.
Now, let's focus on a single example from that dictionary:
Many people expressed concern over moves to restore the death penalty.
I can understand this, according to the dictionary it's an uncountable noun and hence no article, but my question is what change in meaning occurs if I rewrite the above sentence to
Many people expressed the concern over moves to restore the death penalty.
Many people expressed concerns over moves to restore the death penalty.
Best Answer
As a standalone sentence this is unnatural. Upon hearing this, the natural response would be:
It's only if a particular concern had been previously mentioned that the use of the definite article would be natural:
Your second sentence, using the plural concerns and no article, is fine as a standalone sentence.