What is the significance of the exclamation mark at "And 'barn' is only a noun-no verb or subject has entered into the sentence yet!"

The exclamation mark shows strong feeling and is intended to make an emphatic statement.

What kind of emphatic statement?

http://dictionary.reference.com/browse/emphatic

The exclamation mark, also known as an exclamation point, is a punctuation mark used to indicate strong emotion, surprise, or emphasis. In the given sentence, the exclamation mark is used to add emphasis and convey the speaker's enthusiasm or excitement about the fact that the noun "barn" is being mentioned without any verb or subject in the sentence yet.

To understand the significance of the exclamation mark, it is helpful to understand the context of the sentence. From the provided information, it seems that the speaker is emphasizing the simplicity or straightforwardness of the word "barn" as a noun. The exclamation mark helps highlight this point and adds emphasis to the statement.

In general, when encountering punctuation marks like exclamation points, it is crucial to consider the surrounding context and the writer's intent to fully understand their significance.