Rand put together a very thought provoking piece on the threats Google faces.
Since he asked for feedback on the article about other potential threats, I’ll say his list is a fine starting point, then I’ll add these:
The first one I’ll call 1b – A privacy breach event at Google (think search history in a court case or something similar) allows another player to gain share – Ask Eraser could easily benefit from such an event.
The other has to do with monetization. If someone were to come up with a superior monetization model that solves existing problems this could cause people to migrate both advertisers and users alike.