1. Since Mayer was hired, Yahoo stock is up approximately 64%. This is the measurement by which all CEOs are eventually and finally judged. Whether that trend will sustain itself for the long term has yet to be seen, but nobody can see the future. We can only examine what data we have today. And we do have this correlation (acknowledging that correlation != causation).
2. There is a buzz about Yahoo. That hasn't happened with Yahoo's prior CEOs for a long time. For a consumer company, if you have no buzz, guaranteed that consumers won't be interested. Especially for online consumer companies, where customer loyalty seems to switch so easily and quickly.
3. There have been a number of product revamps that have been well received. The biggest one obviously is Flickr. Another one was Yahoo. When was the last time we saw a well-received Yahoo product revamp before Mayer?
Yeah, Mayer's time there has been short. But in less than a year, she's accomplished more than Yahoo has accomplished in several years.