Maybe it didn't make me question EVERYTHING, but it did make me question one of the most influential and invisible elements of life on Earth: what is a world without a gender binary like? How do you think if not in terms of masculine and feminine? What would our world be like if we didn't make assumptions based on centuries of (mostly misinformed) notions of what "belongs" to each gender? Even LeGuin can't escape it in her own narrative; throughout, the androgynous denizens of Winter are referred to as "he," which is inaccurate and limiting in the extreme. But how would we refer to people without gendered pronouns? She never quite figures that part out, but the attempt is brilliant.
This is one of those books that sort of sneaks up on you; I didn't realize just how much it made me think about and question the shaping influence of gender until it was over.