I have spoken with more than a few non-Mormon Christians about their skepticism of Joseph Smith's claim to have seen a vision, and much of their concern is rooted in their belief that such claims are absurd and/or their disagreement with some of the doctrines he subsequently taught. My response generally centers on the example of Paul, the early Christian apostle - whom they all accept as a prophet of God.
1) I don't see any difference in unbelievability between the accounts of the individual visions. Both of them were "fantastic" at the time - and the VAST majority of people who heard Paul didn't believe a word of it either, even as skilled an orator as Paul was. His travels and teachings are quite well-documented; his vision is his word alone. Period. In those ways (nature and acceptance level), there is no substantive difference between Joseph's and Paul's claims.
2) There is serious disagreement among scholars as to whether or not any of the important details in the Bible (especially the Old Testament, but even the New Testament) are accurate to any measurable degree (the details, not some of the large events) - and that's not limited to just the miraculous claims. Therefore, it is FAR more debatable that the men described in the Bible were anything like their descriptions than it is about Joseph Smith - both the apologetic and the dismissive descriptions.
3) Paul was a hardcore fanatic. He went from an extreme, Christian-killing Jew to an extreme, Jesus-preaching Christian. In other words, his basic personality didn't change in the slightest; only his religious orientation did. To use modern examples, he makes Pres. Benson and Elder McConkie look tame in some ways - even though he also preached charity and was instrumental in some of the biggest changes in the early church. In many ways, Elder Packer is probably the closest we have now to a modern-day Paul - since he has said some really conservative, exclusionary things but also some really progressive, compassionate things. I am pretty sure Paul would be the focus of many posts written by mainline, liberal Protestants if he were a modern apostle - and I'm pretty sure many of them would not be flattering. (I'm also pretty sure many evangelicals would not praise James - or even John, the Beloved - if his writings were presented outside the Bible.)
My main point in this post is that Joseph Smith's claims and words are much more similar to Paul's than most people realize - particularly in how those claims and words were perceived in each time period by those who heard them. My secondary point is that I'm sure many of the people who complain the most vocally about Joseph Smith would be complaining just as vocally about Paul - if he had lived in Joseph's time and not so long ago.
Crush the Dead Leaves Under Thy Feet
19 minutes ago