In my view she is one of the most important and misunderstood photographers of the 20th Century. Often decried as a voyeur with an obsession with freaks, I think she was a compassionate truth-teller disgusted by the hypocrisy of social acceptability.
This was the first picture of hers I ever saw.
We cannot ignore her pictures and as a result we cannot ignore their subjects.
Like most of her work it filled me with unanswerable questions:
Who is this boy? Is there something wrong with him? Why did she take this picture? Why do we make toys that resemble hand grenades? What is this picture for? Why can’t I look away?
Arbus stated that: “A photograph is a secret about a secret, the more it tells you, the less you know” 
How true. It’s a statement constantly reiterated by her own pictures. The more intimate they are, the more they show us about the sitter, the more questions arise.
What does Arbus think of her subjects?
What do they think about her?
Why are they willing to share themselves with us?
How are we supposed to react to them?
Is Arbus judgmental or compassionate?
Are her subjects alienated or united?
These questions run round & round us and dog every discussion of her work.
In her 1972 book On Photography Susan Sontag described Arbus' photography as a process of alienation:
Suggesting a world in which everybody is an alien, hopelessly isolated, immobilized in mechanical, crippled identities & relationships." 
Looking at her pictures can often feel like being assaulted, but this is the relationship that Arbus has set up between us and our own prejudices. She confronts us with the reality of our world. I believe that the discomfort we feel in viewing Arbus’ images comes from the disjoint between the message of the photographs and the fictions we tell ourselves as a society about who we are.
All Arbus demands is that we see these people. She will not let us avoid it. Whatever we feel about the people being viewed, we must admit that they exist. They are here in our world and it is up to us to accept that & figure out how we feel about it.
I personally think how you react to these pictures says more about how you see the rest of humanity than it does about Arbus’ views.
The message that I get is that we are not all average looking, healthy, wealthy, conforming, accepted, glossy people. We come in all sorts of shape & size and live all sorts of interesting lives. There are many ways to find contentment.
The subjects are OK with themselves. It is us that bring discomfort into these pictures.
We seek confirmation of our world-view in everything we see & her pictures hurt us to look at because they break societies norms & expose us to the complexity of human reality.
But how close this family looks. How comfortable. No barriers between them. No fear. No concern at all of social pressures & the need to conform.
‘Here we are’ they say ‘take us or leave us’.
How many people caught up in our collagen culture dream of feeling so at peace with themselves & with their family?
Living the suburban dream with their big lawn and their picnic furniture this family seems so cold & disconnected. Their child plays alone & ignored, tiny in the midst of the huge empty lawn. The couple don’t smile, lying stiffly in uncomfortable poses. Everything is in its place. Everything is proper and fixed and sterile.
As Graham Clarke put it in his fantastic 1997 book The Photograph:
Diane Arbus makes us uncomfortable because the truth makes us uncomfortable.
Why do we tell ourselves this myth that childhood is a golden age of rosy cheeks and laughter?
What about the fear, the pain, the first experience of loss, of injustice, of mortality?
Childhood is a place of darkness and light. Our own memories tell us as much, yet this picture is still shocking.
I believe all of Arbus’ pictures can be read as a plea not to believe the simplified myth at the expense of the complex truth.
I believe Arbus is rejecting the idea that there is a ‘normal’ majority and a freakish minority, just as she rejected our fairy-tale views of childhood.
We are all weird, there is no formula for a good life, we should embrace our uniqueness & not try to fit an uncomfortable mold.
I'm ashamed to say I can't give sources for all the quotes, references & pictures I've used in this blog. Somehow I seem to have lost my records.
I've given all the titles & dates of the pictures that I have.
Below I've listed all the books I mention directly. Will add more info if I find it.
1 The Photography Book - Phaidon, 1997
2 This quote appears all over the place. According to wikipedia it comes from the article: "Five Photographs by Diane Arbus." Artforum, volume 9, pages 64–69, May 1971.
3 Susan Sontag - On Photography, 1972
4 Diane Arbus: an Aperture Monograph, edited and designed by Doon Arbus and Marvin Israel (published by Aperture magazine 1972).
5 Graham Clarke - The Photograph: A Visual and Cultural History (Oxford History of Art), 1997