Plunking down $1.1 billion for blogging platform Tumblr was the easy part. Now Yahoo has to live up to its pledge "not to screw it up," as CEO Marissa Mayer put it.

It won't be easy. The history of the Internet is littered with examples of once-promising startups that died on the vine after being acquired—including some bought by Yahoo. Mayer has her work cut out for her.