Reddit is a classic example of this approach. When Reddit first launched, it seemed like there was nothing to it. To the graphically unsophisticated its deliberately minimal design seemed like no design at all. But Reddit solved the real problem, which was to tell people what was new and otherwise stay out of the way. As a result it became massively successful. I'm not quite sure Reddit is exactly "massively successf…
YCombinator does seem more successful than you mention though: they've turned out a lot of companies that, while not always huge, are making money. A few of them are even game-changers in their industries, like Scrib, Posterous, Justin.tv, Dropbox.