I believe there are two important lessons here. First of all, truth must be central to all of our communications. The fact that Jacob showed favoritism to Joseph, going so far as to make a special coat for him, was a reflection of Jacob making his truth clear for all to see. If that was how he felt, then that must be how he presents his feelings.
The rabbis are displeased with this, and urge every parent to never show favoritism to one child over another. The great commentary of the Or Hachaim shows how Jacob's clear communication of his special relationship with Joseph prevented another communication: that of the brothers themselves.
The Rabbi explains how, had the brothers felt free to talk things out with Joseph, even to complain to him, they might of gotten past their feelings of resentment. When we get things off our chest, we can move forward. But once their father was clearly on Joseph's side, "they could not speak peace with him." The brothers' complaint would be against their father, and that far they were not willing to go. Hence the communication was broken, and the brotherly relationship was about to be as well.
The Talmud tells how some of the prophets didn't want to use words of praise that Moses had written in the Torah, because they had seen the destruction of the Temple and the exile. Later, The Rabbis of the Great Assembly reinstituted those words of praise. Why, then, asks the Talmud, did the prophets not utter them? "Because they knew that God is truthful, they did not deceive Him."
In other words, they were so committed to truthful communication, that they even limited their praise of God!
Parents are often guilty of inaccurate communication. When a child misbehaves, a parent will often threaten some form of punishment. To encourage good behavior, a parent may offer a bribe. What does the child learn? Not that the behavior is either good or bad, but that their self-interest in the moment requires a change in their behavior.
Threats of punishment or promises of reward, while effective in the moment, are not truthful communication. Yes, I think every parent in the world has used and sometimes needs to use these techniques, but they are not ideal. Truthful communication would be to have the child understand why the desired behavior is correct, on its own merits.
Dr. Robert Cialdini tells of an experiment where children were told to avoid playing with a certain toy either because, "something bad will happen," or without any reason being given. In both cases, most of the children refrain from playing with that toy. When given the same choice of toys a few weeks later, without any warning this time, the children who were originally told to avoid the toy with no explanation were much more likely to continue avoiding that toy. The ones who had been told that playing with the toy would bring a bad consequence, however, overwhelmingly chose that toy the second time around.
The difference being, the first one involved a consequence, and the children refrain from playing with the toy because of self-interest. When no consequence was threatened, the children understood that there was something inherently wrong or bad about the toy, and continued to avoid it even weeks later.
Thus, as parents, when making behavior requests, it is crucial to emphasize the truthful reason for the good behavior, and not some external consequence.
Another aspect of Jacob's truthfulness comes in the ways he made decisions. After all, it's difficult to call Jacob a man of truth when he told one of the clearest lies in the book of Genesis, "I am your son Esav."
But truth does not require perfection, nor does it prevent mistakes. What it does provide is authenticity. We have lots of inauthentic ways of making decisions. People are influenced by what other people do or say, and often delegate their thinking to whatever the crowd is thinking. Most of the time this is okay, but sometimes, especially when it comes to values in modern society, it is dangerous.
I believe that this is the greatness of Jacob. He never looked for the easy way out. He struggled, he doubted, he took two steps forward and one step back, but he never delegated his thinking to anyone else. He sought the truth in every situation, and made his best call.
Again, Hillel's famous saying is brought to mind: "if I am not for myself, who will be for me?" We must make our own decisions, based on our own and best understanding. Then we are authentic, then we have embraced truth. It is the pursuit of truth, not perfection, that made Jacob and his people the force that will change the world.