Depends on the type of DBA, the company's needs and roles and the DBA's interests.
Someone who's more a developer DBA will probably want to know .net better, someone who's more a 'systems' dba (DBA cross sysadmin) probably needs windows server.
There's no one path and no learning is ever wasted.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability