What is the harm in Waldorf education? Why should anyone oppose it?
Waldorf schools usually admit that their practices are based on Anthroposophy, but they almost always deny that they teach Anthroposophy to the kids. Thus, for instance, Michael House a Waldorf school in the UK has said, "Steiner's philosophy, which he named Anthroposophy, can be applied to all walks of life and provides guiding principles for the teachers' work. It is important to note, however, that Anthroposophy itself is not taught to the children." [10-9-2010 http://www.michaelhouseschool.com/rudolf_%20steiner.htm
For a moment, let's accept this disclaimer. How reassuring do you find it? Consider this analogy. Imagine a school that says, "All of our methods are based on voodoo. However, we do not teach voodoo to the children." Would you be reassured? Would you send your child there?
But is it true that Waldorf schools do not teach Anthroposophy? Sometimes Steiner said that the schools must not teach it, but on other occasions he said the opposite. And in practice, the schools do indeed teach it. Generally they do this indirectly (a point I will return to), but sometimes they do it quite directly, as in telling the students that there are only four elements: earth, air, wind, and water. Indeed, various Anthroposophical beliefs are openly conveyed.
Although he vacillated and contradicted himself on the point, Steiner sometimes said that Anthroposophy should be taught to the students in Waldorf schools. Addressing Waldorf teachers, he said, "You need to make the children aware that they are receiving the objective truth, and if this occasionally appears anthroposophical, it is not anthroposophy that is at fault. Things are that way because anthroposophy has something to say about objective truth. It is the material that causes what is said to be anthroposophical. We certainly may not go to the other extreme, where people say that anthroposophy may not be brought into the school. Anthroposophy will be in the school when it is objectively justified, that is, when it is called for by the material itself." 
On another occasion Steiner told Waldorf teachers: "The problem you have is that you have not always followed the directive to bring what you know anthroposophically into a form you can present to little children. You have lectured the children about anthroposophy when you told them about your subject. You did not transform anthroposophy into a child's level."  Bringing Anthroposophy down to a child's level, so that the child can understand, is very different from leaving Anthroposophy out of the classroom.
Parents of Waldorf students often realize, sooner or later, that the schools are conveying Anthroposophical beliefs to the students. "It frustrates me when people ... [claim] that [Waldorf] schools don't teach Anthroposophy to children ... My daughter's books [i.e., class books created by copying from the chalkboard] show that indeed she was taught Anthroposophy, in picture form as well as in written form. `The human being is like a little universe inside a big one. Sun, moon and stars find their likeness in mans head, trunk and limbs'; `The Sylphs, Salamanders, Gnomes and Undines are the earth's scribes'; `The body is the house of the spirit,' etc. If you deconstruct the lessons, the curriculum and the pedagogy, you cannot ignore the fact that Waldorf is a mystery school, a magical lodge for juniors." Sharon Lombard. 
But usually Waldorf schools are more subtle than this. They generally convey their occult beliefs indirectly, subtly. They are circumspect for a couple of reasons:
1) Anthroposophical "knowledge" is often wacky. Embarrassingly so. For instance, "[A]n island like Great Britain swims in the sea and is held fast by the forces of the stars."  Waldorf teachers don't consider such beliefs wacky, but they realize that outsiders would not "understand" such things, so they usually try to conceal them.
2) Teaching Anthroposophy to the students' brains would be nearly worthless. Steiner disparaged the brain and intellect, saying that these have little to do with real thinking and truth. [See "Steiner's Specific". https://sites.google.com/site/waldorfwatch/steiners-specific
] Waldorf teachers want to bring Anthroposophy to the students' hearts and souls. They care much more about how students *feel* about things than how they *think* about things.
Steiner said that the path to spiritual wisdom comes through our emotions: "I ... want you to understand what is really religious in the anthroposophical sense. In the sense of anthroposophy, what is religious is connected with feeling."  Feelings are far more important than thoughts. "[T]hinking is oriented to the physical plane. Feeling really has a connection with all the spiritual beings who must be considered real ... In the sphere of feelings, human beings cannot liberate [i.e., separate] themselves from the spiritual world."  Feel it, kids. FEEL it. If you feel the invisible spiritual beings around us, you will know the truth.
So, to summarize: Do Waldorf schools teach the kids Anthroposophy? Arguably, no. As ideas, as concepts, as mere fodder for the brain no, they usually do not teach it. But as feelings, as attitudes, as an orientation, as a deeply felt (and unexamined) disposition, absolutely, yes, they teach it. They immerse children in a well-nigh impenetrable fog of Anthroposophical attitudes and feelings for day after day, week after week, year after year. Steiner told Waldorf teachers: "As Waldorf teachers, we must be true anthroposophists in the deepest sense of the word in our innermost feeling."  And the same holds for Waldorf students. Who cares what they think? But as for what they should *feel*: As Waldorf students, you should slowly drift toward becoming true anthroposophists in the deepest sense of the word in your innermost feeling.
What is the harm in Waldorf education? Waldorf schools lead students toward occultism specifically, Anthroposophy. They usually don't jam Anthroposophy down the students' throats; they usually do not spell out Anthroposophical doctrines as intellectual propositions. But as feelings, yes, they convey Anthroposophical feelings and attitudes. The schools do their best to convert children and, often, their parents to acceptance of Anthroposophy in their hearts and souls, if not entirely in their brains. 
 Rudolf Steiner, FACULTY MEETINGS WITH RUDOLF STEINER (Anthroposophic Press, 1998), p. 495.
 Rudolf Steiner, EDUCATION FOR ADOLESCENTS (Anthroposophic Press, 1996), p. 60.
 Sharon Lombard, "Spotlight on Anthroposophy", CULTIC STUDIES REVIEW, Vol. 2, No. 2, 2003.
 FACULTY MEETINGS WITH RUDOLF STEINER, p. 607.
 FACULTY MEETINGS WITH RUDOLF STEINER, pp. 44-45.
 Rudolf Steiner, PSYCHOANALYSIS AND SPIRITUAL PSYCHOLOGY (Rudolf Steiner Press, 1990), p. 70.
 FACULTY MEETINGS WITH RUDOLF STEINER, p. 118.
 But along the way, at least some Anthroposophical ideas do make their way into the students' brains. As one former Waldorf teacher has written, "[S]cience, social studies, and history theoretically were all explored and integrated into the curriculum, but always on a 'Waldorf' timeline and scale, and never in-depth. Additionally, the information imparted was often not accurate. For example, the children were taught that there were 4 elements Earth, wind, fire and air, and that the continents were islands floating on the ocean...." [See "Ex-Teacher 5" http://sites.google.com/site/waldorfwatch/ex-teacher-5