"...also alleged to have stolen Microsoft’s “Activation Server Software Development Kit,” a propriety system used to prevent the unauthorized copying of Microsoft programs."[1]

And another expansion on what the leak could allow:

"According to the reports, not only was Windows 8 leaked, but he also leaked Windows 7 files and the Microsoft Activation Server Software Development Kit which when reverse engineered, could allow hackers to crack the Activation process within Windows, meaning that pirated copies of Windows 7 could continue to function without the nagging presence of popup messages warning users about their copy of Windows."[2]

[1]: http://www.seattlepi.com/local/article/Ex-Microsoft-employee...

[2]: http://www.ubergizmo.com/2014/03/microsoft-employee-responsi...