This is just pure speculation, but here are some of the potential risks I see in this deal:
The number of major portals goes down to 2, which means less choices for the consumers, and less search engines to find our sites. Some of my sites do well in Google, some in Yahoo, some in Live, I like the way the different search engines balance the market.
Major search portals going down to 2, should be bad news for type-in domains. The more power the mega portals gain, the more people will use them to find internet content, the less type-ins.
Plus more market share for Microsoft, known for its bully tactics, buggy software and lack of user-friendliness. Bad news for everyone....
....except Yahoo stockholders! Up 62% in a day, that should give a nice warm feeling..