tedster - 3:35 pm on Nov 3, 2010 (gmt 0)
It's not the crawling - that only takes a small slice of Google's computing power. It's all the things you need to do make an algo work, including all the internal meta-data that needs to be created and attached to all that crawled content. And then, there's crunching all that meta-data to give fast search responses.
I also tend to think writing an algo that is difficult to spam is a very big challenge.