I'm not sure that I would consider this a classic, but I'm also not fully aware of the original impact it had on the literary and real world. The reason is that the straight forward story of a diplomat trying to convince a world to join his planetary alliance just isn't strong enough, but also don't think that this is the story Le'Guin wanted to write. It's just the vehicle for her analysis of human behavior/society and sexuality. 
 I think the world building, with inserted lore between chapters of the main story and other tempo changing tricks made it an amazing read and I'm likely to re-read it at least once as it's the kind of book that leaves you with the impression that you haven't gotten everything when you've read the last word on the last page. 
 If you haven't read it, it's well worth a read. 
 PS. Just found the progress notes from my read and had to add a comment I made towards the end, just to show that even great writers make mistakes. "The reference of a thermometer going counterclockwise made me realize how absurd the Fahrenheit unit is for Winter."