Some empirical feelings, such as sympathy, are indispensable parts of certain moral virtues.

As I understand it, Kantian constructivism is partly a position in normative ethics and partly a position in metaethics. In metaethics, it is the position that ethical claims have truth values, but their truth conditions consist not in a set of objective facts to which they correspond, but instead in the outcome of some procedure of deliberation resulting in decisions about what to do.

As with many metaphysical and religious questions, Kant thinks they lie beyond our power to answer them. If you can't stand the frustration involved in accepting this, and insist on finding some more stable position which affords you peace of mind and intellectual self-complacency, then you will find Kant's position "problematic" in the sense that you can't bring yourself to accept it. You may try to kid yourself into accepting either some naturalistic deflationary answer to the problem or some dishonest supernaturalist answer.

Popular religion since the time of Kant and Fichte has gone in a direction they tried to prevent and that has been disastrous for the humanity both of believers and of the rest of us. Look at the role of religion in Republican presidential primaries if you need any confirmation of this last statement.

As Kant says, the contribution of any common laborer would be greater than that of the greatest philosopher unless the philosopher makes some contribution to establishing the rights of humanity.

There is a very common, though also very silly, picture of Kant according to which as empirical beings we are not free at all, and we are free only as noumenal jellyfish floating about in an intelligible sea above the heavens, outside any context in which our supposedly "free" choices could have any conceivable human meaning or significance. Part of the problem here is that Kant faces up honestly to the fact that how freedom is possible is a deep philosophical problem to which there is no solution we can rationally comprehend.

Marx's own illusion was to think that the working class movement, which he devoted his life to creating and strengthening, would both be socially and politically successful in the industrial nations of Western Europe, and that it would develop an entirely new way of human social life that would retain and even enhance the productive benefits of capitalism while overcoming the inhumanity and exploitation of capitalist social relations. Marx himself had no solutions to these problems. His object of study was capitalism itself.