Much Madness is Divinest Sense

Emily Dickinson rocks. That's all.
...Only not really becuase I don't think I could consider this a blog if that really was all. :)
Anyways, I think the most obvious theme she is trying to convey in this poem is that there is a very fine line between sanity and madness- which is true. People look at Albert Einstien and can argue both sides of him being sane and completely mad.
Besides the obvious reference to psychology, I believe this poem is also interpreting the side of the sociocultural approach to psychology -- the way a person affected according to their surroundings and social interactions / customs.
In the second line, she says, "Much madness us a divinest sense to a discerning eye." I think Ms. Dickinson means that madness is only in the eye of the observer. As Americans, we may see something of a different culture that, because it differs from our customs, seems mad. Other cultures look at the American culture and also see it as completely insane.
Then, "'Tis the majority in this, as all, prevail." -- The majority of those who think something is mad or sane are right. OR the majority will always be the sane / the one's with reason.

"Assent ( I had to look this word up; it means to agree or approve. Cool :)) and you are sane." Basically, conform and agree with the majority, and your intentions and sanity will not be questioned.

"Demur ( Again - I looked this up, but I assumed it meant disagree. I was right. :)) you're straightway dangerous and handled with a chain." If you disagree with the majority, you are seen as crazy and in dire need of help.

I think the theme of this poem is how conformity is seen as "normal." If you step outside the norm, you are "handled with chains" and basically seen as insane. It's a haunting poem that makes one think in what ways one has conformed to society -- at least I did.  I'd say this poem is kind of ike 1984 but with less brain-washing. :)