At first the people of the synagogue in Jesus's hometown of Nazareth seem impressed—more than impressed, actually: "astounded" (exeplēssonto).1
We are not told exactly what "wisdom" he shares with them, but we can surmise that it was something unfamiliar and new, something they have not heard before. Why else would they be "astounded"?
This feeling quickly fades, and Jesus begins to realize that this congregation is not like the one he encountered in Capernaum (Mark 1.21-28). They too were astounded (the same verb exeplēssonto is used of them as well). But the people in Capernaum were ready for something new. They recognized that although he brought a "new teaching," he taught it with "authority."
The people in Nazareth are not so receptive. They liked what they heard—at least, at first—but it will take more than the preaching of a local woodworker to get them to change their views.2
So they turn on him, as people tend to do when their religious convictions are challenged. Jesus responds with a familiar proverb, as he knows this is an old story: "Prophets are not without honor, except in their hometown, and among their own kin, and in their own house." He added that last part himself, perhaps reflecting a little bitterness at the fact that his own family doesn't believe what he's been trying to tell them (cf. Mark 3.20-35).
But what was he trying to tell them?
Like I said earlier, Mark doesn't tell us. But I suspect it might be the same message Jesus began his ministry with: "The time is fulfilled, and the kingdom of God has come near; repent, and believe in the good news" (Mark 1.15).
If this doesn't sound offensive to us today, it is only because we have domesticated the gospel message. We have made it safe so that we don't have to live up to its challenge.3
We have decided that what the "gospel" requires of us is only to believe in a story about Jesus, about how he died for our sins, and how we are saved if we believe this. But obviously this is not what Jesus meant by the "good news," as he was preaching it long before he died.4
We have decided that the call to "repent" (metanoeite) means something like "feel remorse for our sins." While it can mean that, that's not what Jesus meant by it. The repentence (metanoia) that Jesus calls for is something far more profound; it demands transformation, not only belief.
Transformation is hard; we tend not to like it. We resist any understanding that will require it. We find ways of justifying our resistance: "Isn't this guy only a carpenter?"
Today's reading reminds us that we need to stop doing this.
 This verb does not necessarily have a positive connotation. It appears also in Luke 2.48 (where it is translated "astonished" in the NRSV), to describe the twelve year-old Jesus's parents when, after searching for him for three days, they find him at the Temple. Obviously they were not pleased.
It is also found in Matthew 19.25 to describe the reaction of the disciples when Jesus said, "It is easier for a camel to go through the eye of a needle than for a rich man to enter the kingdom of God." The disciples were "greatly astounded," and asked, "Then who can be saved?" It seems to me that this was a worried reaction, not a positive one.
 "Woodworker" translates the Greek tektōn, usually (as in the NRSV) translated "carpenter." The author of the Gospel of Matthew changed it to ho tou tektonos huios, "son of the carpenter" (Matt 13.55) perhaps reflecting embarrassment over Jesus's occupation. Some manuscripts of Mark have been assimilated to Matthew's version, possibly for the same reason.
This may seem odd to us today, but embarrassment over Jesus's occupation was real: Celsus, a 2nd century philosopher and opponent of Christianity, had mocked the church for following a lowly tektōn. Origen responded to this by denying that Jesus was ever described as a tektōn in any of the gospels (Contra Celsum 6.36), which would seem to indicate that Origen was only familiar with the variant reading of Mark 6.3.
 Some Christians might argue that we can't live up to the challenge, and that this is the real point: to show us what hopeless sinners we are. But this is a cop-out; Jesus did not demand the impossible.
 It is also true that the gospel Jesus has been preaching cannot have anything to do with his identity as "Messiah," which won't be revealed in Mark's Gospel for another couple of chapters. And even then, Jesus orders his disciples not to tell anyone about him (Mark 8.29-30).