It's a focus on the core functions. With the iPad, Apple looked at what most people did with their computers most of the time - music, video, games, social networking - and concentrated on that, ignoring all the other stuff. The furore and the criticism has shown that this kind of thinking is rather uncommon amongst geeks, the endless "what about x" arguments, but most people don't care about x. The mass market is remarkably consistent in it's desires.
Google were the first big player to just do search. They didn't aspire to be a directory or a gateway or a hub, just a search box on a page. All the other major search providers had (and most still have) a page cluttered with all sorts of crap. They were obsessed with adding features and increasing revenue at the cost of their core product. A page with nothing but search is a convincing marketing message about how confident Google are in their search technology.
Gmail focussed on how people used email to communicate and organise their lives. Google realised that if you had emails going back and forth in a conversation, it made sense to put it on one page. They realised that having a memory limit was a convenience for the provider that inconvenienced the user. They realised that most people struggled to find and organise things in their inbox.
I don't think any of this stuff is particularly difficult or radical, it just requires a monomaniacal focus on the user's needs. Most businesses think from the inside out and do what is convenient or logical for them, rather than thinking from the outside in and thinking about what the user wants. The conversation on HN seems to make this clear to me - we spend endless time discussing new technologies, new methods, about profit and marketing and the latest buzzword, but when was the last time you read a thread about users?