I guess we can't say this was unexpected. Though I have been critical of Yang in the past, I always maintained that he was only doing what he thought best for Yahoo. He can't really be blamed for the downturn in the online advertising market (everyone is hurting) or even losing market share in search to Google (Semel can take the brunt of that hit). The failure of the Microsoft deal will squarely fall on his shoulders but if it turns out a new CEO can turn Yahoo's fortunes around, the pain of that gaff will be greatly diminished.
So who would make a good CEO? I think they should go and poach a very senior executive from the Google ranks. It would have to be someone from outside the inner circle but still high enough to have had their fingers in a significant amount of Google's operations. My short list would be Shona Brown, Jonathan Rosenberg, David Eun, or perhaps even Marissa Mayer. If Facebook has shown it can poach, why not Yahoo? It has much better financials and a stronger foundation - i.e., better chance of success, in my opinion.