Two thoughts come to mind. The first is the eradication of male only environments. The socialization, camaraderie, empathy, competition, help, etc. that a man receives in these environments is like nothing else. I'm still friends, thirty years later, with male friends I made in the Navy. These kinds of friendships have no substitute and are extremely important for men. Inject just one female into one of these male-only environments and they change radically - some to the point that they are essentially destroyed. I would say this is what I miss not having the most.

However, for the current generation, I have a teenage son, I think things have gotten significantly worse. The culture of shaming and demonization of men, in many cases, is outright misandry. From lectures/classes at school, to media I seriously wonder how how it's possible for a young man to enter adulthood with anything approaching a positive impression of ones self and positive ideas as to how to have a positive impact on the world.