I always found her writing extremely angry. Apparently, her first book, "We the Living" was autobiographical, and she actually did escape Russia during the Bolshevik revolution. She was vehemently anti-communist, which no doubt helped keep her in favor on this side of the ocean during the sixties, etc.
WM mentioned something about her sex scenes; I found that they were always violent, including in her first book, and I suspect that she had been a victim of rape and generally was angry about sex as well as everything else. Certainly there was an undercurrent (well, not always so "under") of anger in all her work, and of protest and in the persistently unfailing achievements of her main characters.
I think that her "Capitalism, the Unknown Ideal" was non-fiction, but doubt that her writing style would have been much improved above her fiction
writing. I had that book at one time, but also had difficulty getting into it and ultimately decided that it probably wouldn't be worth the effort.