So one thing to keep in mind is that during the development and rise of Mormonism were two other trends in American life - the Second Great Awakening and utopianism.

The Second Great Awakening was a religious movement, as preachers (usually Protestant, mostly Methodist or Baptist) would rile up their congregations through revival meetings. Joseph Smith grew up in an area of New York called the 'burned over district', called that because basically so many preachers delivered fiery sermons about the flames of hell in those meetings. Utopianism was vaguely related; and largely revolved around trying to create a new society. People who believed in the particular ideals of the utopian community would gather together and try and build that society. If you've seen the M. Night Shyamalan movie "The Village," that's basically what the utopians were trying to do - a highly regulated society that nevertheless was able to meet the needs of all members.

So, in the context of a religious awakening and also people trying to build their own communities around various shared ideals, Joseph Smith wasn't thaaaat out of place. Things only started to get weird when he began advocating for polygamy. Also, many early Mormons who collected in Nauvoo, Illinois had a distressing (to the other residents) habit of bloc voting. That is, all the Mormons would vote the same way, which indicated some level of political corruption to non-Mormons (fun fact! This is all totally documented in Sir Arthur Conan Doyle's 'A Study in Scarlet').

As far as ethnicity and previous religious belief: the early converts came from upstate New York and the Midwest; which at the time was primarily white & Protestant. So, drawing from that population, most early Mormons were also whites from the Protestant tradition, though there are a few recorded instances of black men being admitted into the Priesthood early in church history (racial exclusion wouldn't occur until Brigham Young took over after Smith's death). And they came from the popular congregations at the time, which were mostly Methodist but could also be Baptist or Congregationalist.

Some first-generation Mormons were also English/Anglican. The first missionaries were sent to England to try and convert the people there. These missionaries were moderately successful in that they persuaded more than a few Englishpeople to convert. Financially, they would have had to have been well-off, since part of the conversion process involved them emigrating to America and settling with other Mormons in either New York, Illinois, Missouri or Utah (depending at which point in time they converted & emigrated).

Some poor families did convert, but many could not afford to make the trip out to Utah when Brigham Young led the Mormons across the plains around 1848. So they probably stayed behind with Emma Smith and became members of the Reorganized LDS Church, now called the Community of Christ and centered in Independence, Missouri (the last place the Mormons lived as a group before going West).

One last attempt to answer your question about why people took Smith seriously: because they wanted to. The Book of Mormon plays into American exceptionalism like whoa, and also answers a question that has plagued Christian theologians since 1492: why didn't God provide for the Native Americans to hear the word of Jesus and thereby find redemption? The Mormon answer is that he did - Jesus totally came to the New World after the Resurrection and set up a Christian society that was later destroyed by their wicked rivals.

Source: I'm a descendant of one of those first generation Mormons, grew up knowing half the story, and learned the other half when I left the Church.