Although this article suggests “infinite” reasons for Google’s success, I’d say there are only two that have made Google worth about 123 billion dollar bills.
The article supports that there have only been two truly notable reasons: A superb PPC model of advertising combined with the most relevant= best search engine to date.

Both the engine and the ad model were largely built by the time of huge expansion.   The story is nicely chronicles in John Battelle’s “The Search”, which also notes how Google’s ad model came about somewhat serindipitously, and basically as a copy of the Goto.com model developed by Bill Gross.  This serendipitous refinement of good ideas  lies at the heart of many great innovations and challenges the idea that greatness comes from stable, consistent, well organized forces of change.

Sure Google has the best technologists, leadership, and corporate culture, but it was the PPC model that was necessary for the success and that is largely ignored in most external analyses (Google knows this all too well).

Good points that without relevancy you’ll lose the audience and the PPC revenues. *Together* these two factors lie at the heart of Google’s success and both are unstable territory, so all are in for more fun in the search sun.