And by 'most significant', I mean it's a heavily US-weighted popularity contest.
Lou Anders checks off his progress on the 'most significant' SF and F books of all time, published between 1953-2002, according to the Science Fiction Book Club. Where are your China Miéville fantasies? Where are your Viriconiums? Where are your Iain Banks novels?
Some good stuff on there, though. But that's the thing with these lists, isn't it? We love to argue about it all.