Terrifying non-Euclidean geometry challenges the sanity of visitors to the ancient cyclopean cities of the elder races in Lovecraftian fiction. You and I might realise that any geometry on the surface of a sphere, such as the planet Earth, is non-Euclidean. By this logic the average globe, popular in studies and libraries of Lovecraft's contemporaries, should have been a mind-rending sight; but how many of his readers at the time had sufficient mathematical knowledge? Euclid lived around 300 BC, the knowledge was out there and there are notable mathematicians from the period, but a high standard of education not available to everyone was likely required for this kind of knowledge.
For those of us who are fans of his work now, it adds to the charm of his works. In many ways he is not a great writer. His uses of exposition and statement rather than description fall short of the standards many writers would aspire to these days. His imagination is what draws the reader in and captivates his audience. It's the grown-up version of the silly monster stories you told to your friends when you were a child and that is a very compelling thing. It does however set him apart from what we might consider more serious fantasy and science fiction authors.
Take War of the Worlds; even in the face of more than a century of scientific discovery and advances in education H. G. Wells' tale retains sufficient plausibility to capture the imagination in a very real way. He made as much a scientific statement as a good story; using his understanding of the science of the period to inform his narrative. Another good example is Arthur C. Clarke, an author whose stories have literally come true in some senses as many of his predictions for the application of technology came to pass in surprisingly accurate forms; such as NASA's Spaceguard Survey and global communications satellites. It's easy to see therefore why these authors are considered masters of the genre.
It's also easy to see how a modern science fiction writer (or fantasy writer to some degree) faces a challenge; the better our education as a society the harder we are to convince. You need to work a little harder to retain plausibility in the face of a readership that understands basic physics. We're all aware to some degree of the difficulties of superluminal travel for example as it has become something of a scientific 'grail quest'. I think this only adds to the achievement of a modern science fiction writer who spins a tale we can believe in. It's simply harder to pull us in and convince us of your future vision and that's something that I can appreciate.