1. It (Mahayana+Zen) postulates Buddha-nature which is contrary to anatta
2. It (Mahayana+Zen) asserts our original enlightenment which makes no sense - why practice then?
3. It (Zen) dismisses sutta study
4. It (Zen) is anti-conceptual and promotes no-thought
I've just come across an article by Charles Muller, a Professor at Tokyo Uni and well-known as a translator and commentator on Korean Buddhism that addresses these points pretty well.
For those who are interested:
from the article:
The most important contributions made by the Ch'an movement are, rather than being philosophical, of a practical nature, in that the Ch'an masters showed a special level of sensitivity to the tendency of the human mind to become enmeshed in conceptual positions. For them, the main obstruction to the attainment of enlightenment had nothing to do with either a lack, or excess of knowledge of the doctrine. The problem was understood instead to be that of the propensity of the mind to become conditioned and attached to concepts. Regardless of the extent of one's doctrinal mastery, such expertise, if not handled properly, will soon turn into an impediment. Therefore Ch'an masters have been noted for their caution when discussing the matter of enlightenment, knowing how easy it is for students to get stuck on words.
But since human beings must inevitably discuss things in the course of teaching and learning, concepts cannot but be established. Having been established, it is inevitable that they will be reified, and clung to. Therefore the need of methods to break such attachments. One of the primary remedies used in this work, is to subject such concepts to an analysis that shows them, just like all the objects to which they refer, to be dependently-originated, and therefore, lacking in self-nature. For the scholar, this view of dependent origination is usually noted, and categorized as a seminal aspect of the Buddhist doctrine. For the Buddhist meditator, the purpose is quite different. The mere learning of such a metaphysical theory in itself will be of little help to the meditator in his fundamental task of overcoming his habituated, mistaken perception of reality. Therefore he engages himself in the practice of meditation, where the observation of the dependently-originated nature of things is sustained for long periods of time, is deepened and enhanced, such that it begins to affect his worldview and actions even while not engaged in formal sitting meditation. Buddhist texts tell us that the result of such a sustained contemplation can be, if the power of the contemplation is strong enough, a major rupture of the habituated discursive process, which allows the disclosure of deeper aspects of the consciousness.