Experience with large scale deployments using Chef, actually authoring Chef cookbooks/recipes...not simply administering hosts that use Chef.
Experience with large scale Linux systems administration.
Expert-level programming ability in shell scripting.
Would be nice any of these, along with the above:
Experience with: Hadoop, CDH (Cloudera's Distribution of Hadoop), Kerberos, LDAP, Active Directory, TLS certificates, Ansible, expert-level Perl scripting, REST API programming, DBA experience with MySQL.