As a former Yahoo employee, I would say the answer is obvious: talent. Yahoo had an inconsistent talent bar: some teams had a very rigorous interview process and managed under-performers out, others did not do either.

Autonomy requires trusting your employees to do what's necessary even without direct supervision. That's not to say there weren't many strong engineers at Yahoo: indeed, I can think of one or two who have only remained at Yahoo due to ability to work from home -- so Mayer's policy will send them scuttling, while retaining those who are only at Yahoo because they can't find a comparable position (even sans the ability to work from home) elsewhere.