What does a SQL Server DBA need to know about Windows Server?
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...