One thing for me was how light hearted and funny Lenin was. I expected it to be really hard going like a lot of Marx but he had such a wit about him and I remember laughing out loud a few times by his turn of phrase. I don’t think he gets enough credit for it, he took a really serious, dry subject and made it enjoyable to read. He really was an incredible writer.
The rigor of Marxist philosophy of science and epistemology, and how the academy has actively ignored these great thinkers because they would provide a viable alternative to the status quo. Like Engles’ “The Dialectics of Nature.” And Mao’s work “On Contradiction.”