First, read this definition of unschooling:
(I know it's long, but to understand what I am dealing with, try to read it all)
From Wikipedia, the free encyclopedia
Unschooling is a form of education in which learning is based on the student's interests, needs, and goals. It may be alternatively referred to as natural learning, child-led learning, discovery learning, autodidactic learning, or child-directed learning.
Unschooling contrasts with homeschooling in that the student's education is not directed by a teacher and curriculum. Although an unschooling student may choose to make use of teachers or curricula, s/he is ultimately in control of his/her own education. The student chooses how, when, why, and what s/he learns. Parents who unschool their children act as "facilitators" and provide a wide range of resources, instruction and support. Unschooling begins with a child's natural curiosity and expands from there, as an extension of his/her own personal interests and needs.
The term unschooling was coined by John Holt. An author of ten books on education, John Holt founded the unschooling magazine Growing Without Schooling.
Unschoolers commonly believe that curiosity is innate and that children want to learn what is necessary to become an adult. Some argue that institutionalizing children in what they term a "one size fits all" or "factory model" public school is an inefficient use of those children's time because it requires every child to learn specific subject matter in a particular manner, at a particular pace, and at a particular time regardless of that individual child's present or future needs, interests, or goals, or any preexisting knowledge he or she might have about the topic.
Many unschoolers also believe that "...the anxiety children feel at constantly being tested, their fear of failure, punishment, and disgrace, severely reduces their ability both to perceive and to remember, and drives them away from the material being studied into strategies for fooling teachers into thinking they know what they really don't know." Proponents assert that individualized, child-led learning is more efficient and respectful of children's time, takes advantage of a children's interests, and allows deeper exploration of subjects than what is possible in formalized education.
Unschoolers often believe that learning any specific subject is less important than learning 'how' to learn. They believe, in the words of Alec Bourne, "It is possible to store the mind with a million facts and still be entirely uneducated," and in the words of John Holt, "Since we can’t know what knowledge will be most needed in the future, it is senseless to try to teach it in advance. Instead, we should try to turn out people who love learning so much and learn so well that they will be able to learn whatever needs to be learned." This ability to learn on one's own makes it more likely that later, when the child is an adult, s/he can continue to learn what s/he needs to know to meet newly emerging needs and interests. S/he can return to any subject that s/he feels wasn't sufficiently covered, or learn a completely new subject.
Another belief frequently held by unschoolers is that "Children... if they are given access to enough of the world, they will see clearly enough what things are truly important to themselves and to others, and they will make for themselves a better path into that world than anyone else could make for them."
Some people unschool for not only educational reasons, but social reasons as well. They think that the age segregated and tightly controlled environment of schools creates an unhealthy social environment. They feel that their children (or they themselves if the student is the one who initiated the unschooling process) benefit from coming in contact with people of diverse ages and backgrounds in a variety of contexts. They also feel that their children benefit from having some ability to influence what people they encounter, and in what contexts they encounter them. Unschooled children are often reported to be more mature than their schooled peers,  and some people believe this is a result of the wide range of people with which they have the opportunity to communicate.
According to the article Homeschooling: Back to the Future?, unschoolers have been admitted to most universities (including Ivy League schools). The article states that "in the absence of a transcript or high school diploma, applicants can submit samples or a portfolio of their work, letters of recommendation, and CLEP and Stanford Achievement Test scores." Some universities consider unschoolers to be an asset because they tend to love learning, be self-motivated, and know what they want to get out of their college experience. According to Johnathan Reider, an admissions officer at Stanford university, speaking of homeschoolers in general, "The distinguishing factor is intellectual vitality. These kids have it, and everything they do is responding to it." 
Some common arguments against unschooling are given below.
Some children lack the foresight to learn the things they will need to know in their adult lives.
There may be gaps in a child's education unless an educational professional controls what material is covered.
Because schools provide a ready-made source of peers, it may be more difficult for children who are not in school to make friends and develop social skills than it is for their schooled peers.
Because schools provide a diverse group of both adults and students, it may be more difficult for children who are not in school to be directly exposed to different cultures, different socio-economic groups and different world views.
Children have a vast capacity for learning new things, so it is the responsibility of adults to ensure that they learn a number of essential things, as it could be more difficult to learn those things as an adult (what these essential things are varies from critic to critic).
Some children are not motivated to learn anything, and will spend all of their time in un-educational endeavors if not coerced into doing otherwise.
Not all parents may be able to provide the stimulating environment or have the skills and patience required to encourage the student's curiousity.
Because they often lack a diploma from an accredited school, it may be more difficult for unschooled students to get into college or get a job.
(Feel free to research it yourself for more details...there's a lot of stuff out there about it, and of course unschoolers would tell you that you can't define it.)
Now, here's what has stuck in my craw:
A few days ago, I visited a blog that I read occasionally, The Contrary Goddess.
I had always thought that what she had to say was interesting: they grow their own food, buck tradition and convention, etc. She posted this, and I thought I would leave a nice comment about "unschooling", and I didn't really understand all there is to unschooling, obviously.
I was not expecting the comments that followed. I got jumped on like a duck on a junebug. (That's a nice bird analogy, isn't it?)
I invite you to go to her blog and read what she had to say, I had to say and her fans had to say.
WARNING: These are self-proclaimed "unschoolers", so leave a comment at your own peril.
I don't hate the woman at all, and I can't blame her proponents for standing up for her. It reminds me of anytime that Julie gets a negative comment on her blog and we all rush to her defense. But you know, Contrary Goddess could have just said, "Thanks for visiting my blog. But there is more to unschooling than that." Or something along those lines. But I basically was told that I haven't reached my full potential because I have been schooled versus unschooled. Okay, okay, I don't "get" it...but Damn! As an educator, I took it a little personally. Don't get me wrong...the idea of letting kids decide what they want to learn about sounds good, but I can't believe that they reach THEIR full potential.
Now, I don't want everyone to go over there and bash HER just because you like ME. I would love to know how all of you feel about this subject. I know there are educators who read my blog, and others who must have an opinion about this.