This is an important question. But to ask what is pseudoscience, we first have to ask what is science?
To qualify as science, something must advance a hypothesis and demonstrate repeatable results or it simply isn't science. If we accept Karl Popper's idea of falsifiability, this means that whatever hypothesis is to be advanced by empirical science that it should be testable and that we can demonstrate that the hypothesis is true of false ("falsifiable") with empirical results. This means that if you claim to have an invisible purple dinosaur friend that no one else can see or experience, this does not exactly reach the standard of the scientific method. That said, just because a hypothesis is not demonstrable does not mean that it is necessarily false, but merely that it it doesn't fit into the rubric of the empirical sciences.
But, then again, who among us submits all our inquiries to a robust double-blind scientific study before making a decision? Our day to day lives cannot use such scrutiny. Instead, we have to work from instinct or intuition based on what our biological inheritance has given us and what concepts experience has granted us.
In this sense, from the view of external empirical science, biodynamics does not quite merit to be called science, nor does it pretend to be. Biodynamics is a leap of intuition. This is often forgotten: the flash of insight usually precedes the experiment. An experiment often demonstrates the veracity of an intuition -- or that the insight was wrong. As such, those utilizing biodynamics are quite open to experimentation. Considera.org has a compendium of research explored within the biodynamic world.
To suggest that biodynamics is a "pseudoscience" is as absurd as saying that art is a "pseudoscience" -- neither art nor biodynamics in it self makes any such claim. As Steiner himself repeatedly said, these insights are suggestions that must be tested.
What hypothesis does biodynamics as an intuition advance? That these special biodynamic preparations will measurably improve the quality of compost, hasten the formation of humus and the build-up of organic matter in the soil, and increase the nutritional value of crops. This hypothesis must be tested, but funding for research is a very different question!