Hi. I'm an orthodox (not Orthodox) Christian. I'm not here to "Mormon-bash," but someone had laid down the gauntlet for a non-Mormon to enter the debate, so here I am.
I think the problem most non-Mormon scholars have with the Book of Mormon, is that it isn't historically or anthropologically defendable. There are no extant ancient texts. Many sites mentioned (including the hill where the plates were found -- there is no hill where Joseph Smith said there was) cannot be geographically placed. Also, a critical reading of the BoM yields no compelling evidence of any literary style corresponding to the time the Book was written.
On the other hand, the Bible is steeped in history and anthropology. While there are some geographical problems, these can be attributed to the metaphorical nature of many Bible stories. There are several recognizable literary devices used, which fall into line with other writings of the same time periods.
The problem, as I see it, is that, by the time the Bible was subjected to critical analysis, it had already been long established as revelatory. When the BoM was apparently translated, that work happened in an age where skepticism, analysis and reason had aready overshadowed simple acceptance of a writing as being revelatory.
Another problem that I, myself, encounter is that the Bible is highly allegorical and metaphorical. The Bible is not to be read literalistically. As I understand Mormonism, the reading of both the Bible and the BoM is more literalistic. That creates a problem at the outset, for me, and for most Biblical scholars.
Then there's the apostacy issue:
Reasonable scholarship of the Bible shows that the canonical gospels were not written by people who actually knew or were disciples of Jesus. The earliest gospel was not written until after the Fall of Jerusalem in 70 c.e. The Biblical canon (the Bible as we now have it) was not set until the third century. If, as Joseph Smith maintained, authority was "taken away" from the Church by God following the death of the last original apostle, then the whole of today's Bible was compiled and edited by apostate Christians and, therefore, not authoritative. By definition, that point alone either 1) makes the basis of the Mormon gospel (as presented in the Bible, which Mormons view as authoritative,) as apostate and as unauthoritative as the rest of Christendom, or 2) The "restored" gospel of the Mormon Church must be different from the gospel of the Bible.