meaning - What does using という add to a sentence?

This sentence is from a grammar website:

主人公が犯人だったというのが一番面白かった。

Why is という necessary?

What does it add?

And what meaning is lost in writing the sentence without it? For example, as in:

主人公が犯人だったのが一番面白かった。

Answer

In The Structure of the Japanese Language, Susumu Kuno's treatment of こと, の, ということ and というの as complementizers separates them into two categories - one involving presupposition and one without.

All the four forms listed above can be used when there is presupposition of truth. But こと and の cannot be used when the predicate does not contain presupposition.

I'm going to attempt to summarise some of that here but his analysis is actually much more complete.

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Observe this example taken from his book:

[1] It is probable that John hit Mary:

a. ジョンがマリを殴った​｛こと・の｝はありうることだ。　(Unacceptable)

b. ジョンがマリを殴った​｛ということ・というの｝はありうることだ。　(Acceptable)

[2] It is false that John hit Mary:

a. ジョンがマリを殴った​｛こと・の｝は嘘だ。 (Unacceptable)

b. ジョンがマリを殴った​｛ということ・というの｝は嘘だ。 (Acceptable)

What is observed is that in the 1a and 2a, こと and の cannot be used because "is probable" and "is false" indicates that the speaker does not presuppose the event of "John hit Mary" to be true.

I shall not include the complete analysis of こと, の and と but in summary:

• こと and の assumes that the event is true, while と does not.

Also summarising the analysis of こと and の:

• の represents a tangible action/state that is directly perceptible (by the five senses), while こと does not and instead represents a more abstract concept.

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So in fact there is a slight difference in meaning.

Comparing:

1. 
   

   主人公が犯人だったのが一番面白かった。 (主人公が犯人だった is presupposed to be true)

   
   


2. 
   

   主人公が犯人だったというのが一番面白かった。 (There is no presupposition) Or (主人公が犯人だった is presupposed to be true)