The word of the Lord to Lehi comes in a simple request to look upon the ball, or director. They do so and are astonished. Why?
In the original discovery of the Liahona, the sudden appearance is miraculous, and the ball is of curious workmanship, but at least in Nephi's report, what the ball did was quite easily understood and stated: "1 Nephi 16:10 And it came to pass that as my father arose in the morning, and went forth to the tent door, to his great astonishment he beheld upon the ground a round ball of curious workmanship; and it was of fine brass. And within the ball were two spindles; and the one pointed the way whither we should go into the wilderness."
All we are told at the beginning is that there are two spindles, and one of the two points the way. Nothing appears particularly exciting about this, and the family simply sets of as directed. In this occasion, however, the nature of the Liahona appears to change. No longer is it a mundane (though admittedly special) compass, but an instrument of revelation.
The surprise of the family noted in verse 27 is explained in verse 29. New writing has appeared on the ball. Verse 29 is clearly written from a perspective long after the finding of the ball, and it might blur the true nature of the event Nephi is describing. While Nephi knows from the intervening years that the ball would operate according to their faith, and that the writing might change from time to time, it is certain that he is describing at this time the very first instance where the ball showed itself to be more than a simple compass. There is no other explanation for the astonishment at seeing that the writing had changed unless this were the very first time that had happened. Nephi's explanation that this continued to happen is an emendation based on his later experiences.
Verse 28 contains Nephi's conclusion that the ball operated according to their faith. This appears to have been an astute deduction on his part, if this is the first time that the Liahona had given such specific instructions. Given Nephi's growing spiritual perspicacity, this is not a far stretch. Nephi sees the changes in the ball, and comprehends the implications that their own actions have upon it. He contrasts their formerly forlorn state with the functioning of the ball after the general repentance and humbling of his father and brothers. His presumption will hold true.
Verse 29 also includes Nephi's homiletic conclusion: "And thus we see that by small means the Lord can bring about great things." Of all of the lessons Nephi might have derived from this incident, he presents this one. He does not wax eloquent about faith (clearly an appropriate topic here), nor about the effect of our pride upon the ability of the Lord to present us with his will. Instead, Nephi suggests that the Lord is able to use small means to bring about great things.
The "small means" was the revelation through the Liahona. While that kind of remarkable transformation of an inanimate object into a communicative one is quite miraculous to us, Nephi nevertheless lists it as "small means." Nibley has used this passage to suggest that the Liahona worked in ways not unexpected (Nibley, Hugh Since Cumorah. Deseret Book. 1970, p. 287). This initial passage suggests that while the basic mechanism might have been more common, this incident clearly was not.
Against what are the "small means" contrasted? In the text it is "great things." What were those "great things"? In the context of this event only, the "great things" would have been telling Nephi where to go to hunt. At the very moment of the instruction, however, even that might not have seemed so great - at least until Nephi returned with food. Does that yet qualify for "great things"?
I would suggest that the "great things" refers to all of the results of following the ball, which is a transparent metaphor for following the will of the Lord. In the Lehite saga, the great things accumulated into the entire marvelous experience of their removal to the New World. In a larger sense, and certainly one implicitly understood by Nephi, our spiritual exaltation is the ultimate definition of those "great things" which may turn on small things, such as listening to a particular simple instrument of God, whether it be a ball or a still, small, voice.