I’ll submit as a defining characteristic of the alternative left vs. the more mainstream progressives is that the alt-left is much more skeptical of the transcendentalist underpinnings of a lot of leftist thought, especially post 1960s. We are, at heart, a push to bring enlightenment rationalism back to the center of leftist thought and activism.
Transcendentalism – the kind originating back in 19th century New England, had its roots in European Romanticism, and is marked by a kind of idealistic utopianism. Religion and science alike were distrusted, in favor of subjective personal intuition and creativity. Nature and people considered “closer to the earth” (indigenous peoples and women) tended to be idealized.
Emotion and intuition were extolled as being a kind of liberation from the bonds of strict empiricism. People were thought of as naturally good but corrupted by overly rationalized institutions and belief systems, against which struggle was seen in idealized terms.
I suspect that the alt left’s appeal is to people with liberal sensibilities but who are off-put by romanticist elements in mainstream progressivism. This is marked by the progressive’s tendency to go beyond mere equal rights for “marginalized” peoples and into flagrant idealization of them.
Women, immigrants, people of color, indigenous people, Muslims, Buddhists, LGBT people and so on are not necessarily viewed as superior to white Christian (or atheist) males (although they can be) but as being less corrupted by the soul destroying rationalism of capitalism, objectivism and/or Christian theology, which is thought to instill in the white male an oppressive, dominating mentality.
I find this kind of thinking to absolutely pervade post modern progressive thought. It is anathema to me, and I think the rise of the as yet embryonic alt-left is kind of rationalist and realist reaction to the romanticist conquest of materialism on the left.