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As I have already stated, the most important aspect is prejudice: whether the Mainstream Christian approaches Mormonism with a positive prejudice, on the assumption or in the hope of finding an underlying unity; or (as is usual) with a negative prejudice, that assumes Mormonism is not Christian, and which puts Mormonism on trial - confronting Mormonism with a set of accusations all of which it must refute on a point by point basis.
In other words, the nature of the prejudice (or prior assumption) will have a vast and decisive effect on the procedure of evaluation and therefore the outcome of evaluation.
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Because Mormonism is approached by most Mainstream Christians with a negative prejudice, the differences between Mormonism and Mainstream Christianity get presented as a shopping list of point-and-sputter factoids: "Mormons believe God (the Father) had a body", "Mormons believe the risen Jesus visited America" etc etc.
Now many of these shock tactics are misrepresentations and de-contextualized distortions - but of course Mormonism does have many and important differences from mainstream Christianity.
Now, if these are examined one at a time, and especially with a negative prejudice, then this list of differences will seem either wickedly defiant; or simply absurd and arbitrary.
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But in fact (and I mean in fact) most of these differences (and all of the really significant ones) emerge from an underlying metaphysical difference - philosophical pluralism - and from a different way of reading the Bible (taking it at face value, minus Classical philosophical preconceptions).
I assume that this different perspective came from Joseph Smith and predated the writing of the Book of Mormon, which was then written in accordance with this mode of understanding so different from the theology of the post-Apostolic era (but comfortably consistent with the Bible as understood by a plain man's reading).
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In sum, Mormonism is Christianity; and differs from other denominations primarily in its metaphysical assumptions (i.e. its philosophical assumptions concerning the basic nature or structure of reality) which are pluralist rather than monist.
These metaphysical assumptions are not a part of Christian revelation, rather they are second order (and historically later) attempts to systematize revelations, and bring them into line with other forms of understanding.
For example, much of the intellectual theological work of the first few hundred years of Christianity seems to have focused on bringing Christian understanding into the framework of Classical Philosophy, in its various manifestations.
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The vicious Christological disputes (disputes concerning the nature of Christ) of these early centuries seem to have been (at least to some significant extent) a consequence of this philosophical work - when it was found that perfectly clear and comprehensible Biblical revelations were difficult - in fact impossible - to fit into a self-consistent philosophical framework which also fitted with revelatory/ traditional understandings of the nature of Christ.
It was probably the insistence (despite the difficulties) on adopting a Classical philosophical understanding, and giving this philosophical understanding primacy over revelation, which probably led some into heresies - as they followed their philosophy wherever it led, rather than giving primacy to the revelations.
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So, Christianity has various metaphysical systems backing-up revelation: most famously Platonism (associated with St Augustine) and Aristotelianism (associated with St Thomas Aquinas).
Since around 1830, to this can be added pluralism/ pragmatism - with Mormonism broadly summarizable as Christianity backed-up with a kind of precognitive version of the distinctively 'American' philosophical perspective described by William James and his colleagues.
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But what is true?
The answer will have to take into account that more than 2000 years has failed to answer objectively whether Plato or Aristotle was true, or even which system was true-er.
Because the truth of metaphysical systems is not an empirical matter, because the metaphysical system includes and defines empirical evaluations.
How, then, to choose which metaphysical system to adopt?
In the first place, the system should be self-consistent.
Having passed this test, and beyond this, the choice of metaphysical systems would take account of factors such as expediency (personal, and social, fruits of the belief), and also comprehensibility, and intuition/ personal revelation.
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Different metaphysical systems work for different people for different purposes and at different times - each has advantages and disadvantages.
All I would point out is that the Mormon metaphysical system very obviously has many and important advantages (in terms of fruits, of comprehensibility, and as validated by personal revelation) for some people at this point in history.
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