Nephi would like to teach us (his readers) all he taught those who lived with him and heard him speak. But he could not. Even the things he was able to etch in the metal record he left was incomplete when compared to the body of teachings he preached to his people.
There is also a significant difference between speaking and writing. When you speak there are many tools of speech - emphasis, movement, presence, and radiation of the Spirit to help the speaker measure the effect of the message on the audience. When Nephi taught by the power of the Holy Ghost, he was able to see how his audience was receiving it. He knew when it penetrated "unto the hearts of the children of men."
Writing was another matter. Particularly when it would be translated from one language to another before the gentiles would receive the words. The distance and language between Nephi and his audience is so great that Nephi came to the sad realization that a reader who is not already prepared to have the Spirit with them as they read will miss the power of the message.
In their presence Nephi could use the power of the Holy Ghost to affect the spirit of those who were listening. However, a reader separated by language and culture, and more than two millennia would have to have the Spirit first before being able to understand his message.
It was the recognition that many gentiles would read this record without possessing the Spirit that made Nephi acknowledge the gap between his spoken ministry and his written one. Those with "hard hearts" may be affected by his presence and preaching. Those with "hard hearts" who only have his written record, however, are going to "cast things away which are written and esteem them as things of naught." They won't recognize that they were from God, written by a prophet who knew God, and were the result of a commission to preach given by God. Instead they will think him "a thing of naught."
Nephi's message will mean far more to those who are prepared. For those who are not prepared, the message will be meaningless. Nothing. A thing of "naught" to be "cast away."
That is always the case. The Lord commissions someone with a message and the audience has a role in receiving the message. Powerful public ministries do not convince everyone. Even Nephi failed to convert Laman, Lemuel and the majority of those who were living together at the time of Lehi's death. Then, immediately upon Nephi's death, there were struggles in the society he helped found.
The process of salvation is always a work between God, His children, appropriately sent messages, and adversity and opposition. Nephi is reminding us how vital having the Spirit is to the success of understanding his written message. We should ask ourselves often if our hearts are open to receiving truth, no matter how it comes to us, and no matter how it may challenge our presumptions, pride and foolish traditions.
What a terrible thing it will be for some to realize they "esteemed as things of naught" the very words which might have saved them had they given heed.