Japanese literature emerged from Soseki, the same way that Russian literature emerged from Pushkin. Soseki's books touch on all the major themes of every major Japanese novelist: Kawabata's aching regret, Mishima's inexplicable drive, Tanizaki's fatal attractions. Even Haruki Murakami's depiction of ennui-laden college life in Norwegian Wood is very heavily influenced by Soseki's Sanshiro. Sure, Soseki doesn't develop any of these themes fully. He's less focused than any of the above authors, he doesn't quite match Kawabata's striking imagery or Mishima's fiery heroics. But everything started with him.
Sanshiro was Soseki's first major work. It is the first installment of his so-called "first trilogy," which also includes And Then and The Gate. The three novels are unrelated, but they seem to fit together because of a neat progression in the age of the protagonists. Sanshiro is about a young man in his twenties. The other two novels are about people in their thirties and forties; they are different people from the titular protagonist of Sanshiro, but they may have been somewhat similar to him when they were young.
So, this is Soseki's "novel about youth." It has a standard coming-of-age plot. A young man from the provinces, unschooled in the ways of the world, goes to college in Tokyo, meets city intellectuals, falls for a mysterious young woman, suffers from unrequited love, encounters situations he wasn't ready for.
But Soseki walks an extremely fine line in depicting Sanshiro. You can really appreciate Soseki's subtlety if you read this book right after Norwegian Wood. The protagonist of Norwegian Wood is also supposed to be "ordinary," with no particular talents, but actually he's immensely charismatic, attracts lots of beautiful women, and never loses his cool, devil-may-care composure while dealing with them, even while he's supposedly suffering. Murakami secretly flatters his audience.
Soseki's Sanshiro is not a ladies' man. He has no idea of how to act with women. When talking to his love interest Mineko, he never understands what she means. He always feels that she's holding back, or that she's laughing at him. He loses face, he feels embarrassed and uncomfortable. It's not that he says the wrong thing, it's that he has no idea what to say at all (although he's just as educated as the next fellow), and is usually reduced to lame one-word answers.
At the same time, Soseki avoids the opposite extreme. If Tanizaki had written this book, he might have been tempted to exaggerate the protagonist's ineptitude, to depict him as maladjusted and neurotic and to suggest that he loves the girl out of masochism. But Soseki insists on Sanshiro's normality. Sanshiro is well-adjusted. People like him. He makes friends easily. He doesn't write moody poetry or dream of decadent eroticism; he doesn't have a tragic past. He just doesn't know about women yet. Soseki shows how Sanshiro is attracted by Mineko's mysterious character, while never actually focusing or dwelling on the content of his thoughts or fantasies. It is a very perceptive move -- in this case, more detail would have completely changed the impression given by the character.
This gives the novel a uniquely fresh and clean feeling. Sanshiro is not only "ordinary," he has an admirably healthy and well-rounded personality. He is susceptible to the influence of others; he's not immune to turbulent emotions and suffering. However, one gets the feeling that he will surely turn out all right in his life.
Not everyone does, however. The novel gives a contrast to Sanshiro in the form of Professor Hirota, an intellectual with a keen sense of irony who often makes philosophical observations or criticisms in casual conversation. Hirota does not benefit from the comparison. It eventually becomes clear that he is basically a gifted dilettante, which is why his low-key, "unrecognized" but hassle-free life is really ideal for him. There is a subplot in the novel where one of Hirota's students tries to get him a better job -- but if Hirota were to get it, he would be inherently incapable of living up to the student's expectations, because it would require him to have a coherent worldview instead of a convenient set of aphorisms.
Mineko is extremely vaguely sketched. Soseki simply doesn't consider it necessary to explain anything about her whatsoever. But then, it truly isn't necessary. It simply does not matter whether or not she loves Sanshiro, or whom she really loves, or what she thinks about, or what she wants. The point is that, whatever it is that she actually wants, Sanshiro would never be able to understand that she wants it, much less give it to her. He is bright, and a good person. But, at this point in his life, he thinks in fundamentally different terms from Mineko. From the start, there was never any way for them to explain themselves to each other, no matter what they might have said.
I should also say that Soseki can be very funny. He never gives more specifics than he has to, so it can be difficult to discern his tone. But his depiction of Sanshiro's first day of classes makes a complete mockery of the curriculum in just three sentences: "First Sanshiro learned that the word 'answer' came from the Anglo-Saxon 'andswaru.' Then he learned the name of the village where Sir Walter Scott had gone to grammar school. He carefully recorded both facts in his notebook." (30) Soseki also gently laughs at his protagonist in his depiction of Sanshiro's fussy, boyish concern over how to conduct himself toward Miwata Omitsu, a girl from his hometown. One suspects that Sanshiro will end up marrying Omitsu in a couple of years, and that he'll probably be very happy with her.