Andreessen, who is on the boards of Facebook and Hewlett-Packard , calls Mayer, a 37-year-old Stanford-trained engineer who was Google’s employee No. 20, “a product-centered CEO.”

If Yahoo had wanted a “sales-centric CEO,” he noted, the board would have chosen insider Ross Levinsohn, who has been interim CEO since the board pushed out Scott Thompson earlier this year. Apparently, most Yahoo employees were betting that Levinsohn would secure the top job.

Will Levinsohn stay at Yahoo? The board hopes he does, especially since Yahoo’s recent reorganizations have placed many of Levinsohn’s followers in senior positions. Retaining Levinsohn will likely require masterful diplomacy by Mayer, who is the youngest woman ever to make Fortune‘s Most Powerful Women list. She ranked No. 38 on the list last year.

The other big question, of course, is whether Yahoo can be revived following unsuccessful runs by a series of CEOs including Thompson, Carol Bartz and co-founder Jerry Yang. “There have been very few web turnarounds,” Andreessen noted, adding that one Silicon Valley turnaround proves it can be done. “Apple showed that tech companies can be turned around…spectacularly,” Andreessen said. Though, he added, the highest standard for CEOs, Steve Jobs, is very, very hard to come by.