What does a SQL Server DBA need to know about Windows Server? Analogous to my previous question What does a DBA have to know about SSAS?, which generated a really wonderful answer (thank you, @ConcernedOfTunbridgeWells!), I now ask a similiar question:

What does a SQL Server DBA need to know about Windows Server in order to be able to manage it efficiently and understand what is going on behind the scenes and to which degree is it important to know the underlying OS in which SQL Server runs to in order to become a great DBA?

It can be individual features, books, from experience, you name it!
You definitely need to know your way around...


Performance Monitor 
Disk Management (mount points, 64k clusters, RAID levels etc)
Group Policy + Active Directory + Security
Shares + NTFS permissions
Cluster Service + resource groups
Powershell
Physical/Logical sockets/cores + affinity
NUMA architecture


Without these (and others) you're ineffective outside of raw SQL Server work (eg setting up logins). As soon as you want backups, one or more of the above starts to apply...