In Gospel Doctrine two Sundays ago, the teacher asked what makes the Doctrine & Covenants different from other scriptures we have. The answers centered on the fact that the Doctrine & Covenants is not a translation, that it is revelation specifically for our day, and that it is an open book of canon -- all very fine answers.
But for me, the most distinctive thing about the Doctrine & Covenants in contrast to other scriptures is that because it is so recent, we have a pretty good close-up look at the revelation and evolution of scripture. We have many primary textual sources for the Doctrine & Covenants, unlike any other book of scripture, for which we have only copies of copies. This can give us insight into the nature and process of scriptural revelation. We get some of this in Biblical scholarship, as more manuscripts are found and compared, but it is all at a much greater distance.
And it seems to me that what we find is that scriptural revelation isn't a very tidy process. A fair number of things that we have in the Doctrine & Covenants come from letters or comments made in a conversation and recorded later on. Expansions, changes, drafts, editing, etc. all contribute to something that challenges the simplistic views of scripture I grew up with, in which God dictated the revelation word for word and the prophet just wrote it down.
The implications of this are fairly significant, I think, and give us some insight into the relationship between God and man and our own need for humility.