Don Reisinger, in his technology column, nails it:
Surely, some will attribute Google's success to its better search results or Yahoo's management troubles or Microsoft's poor offering, but it goes far beyond that. Search isn't simply about relevant results or the competition. Instead, search is all about getting you to your destination as quickly as possible.There has got to be a sermon illustration somewhere in there.
And so far, it's quite apparent that only Google understands that basic premise...
Think about that for a second. Doesn't that run directly against everything we know about the Web? Practically every site is designed to keep you there. How many times have you tried to click links in a blog post, only to find that it links you back to another section of the same site? It happens all the time.
Basically, more Web sites try to keep you from going elsewhere for fear that you will never come back. But Google doesn't worry about that.
Google wants you to leave and feels like it has done its job when you do. Now, part of that equation revolves around the quality of search results and the service's usability, but we can't downplay the fact that letting you go is a key to the company's success. ~ link