1. Ability to use search engines (google in particular). Why?? Because 90% of cases accelerate solution for the problem within ten minutes. Incidentally, it is helpful for not only system administrators but for everyone.
2. Knowledge of English. Why?? Because virtually all of the useful documentation is in English. There are good articles, but still a lot of dross. In addition, the literature is not in the native language, or the quality of translation prevents proper understanding of the content. The developers of various software also typically communicate on their mailing lists in English, not in Hindi or any other languages.
3. Understanding the ISO / OSI and TCP / IP stack. Why?? It allows you to gain skills in diagnosing network problems by several orders of awareness of potential problems and opportunities of network protocols, as also to properly configure the routing and packet filters. You may not know / be able to solve these problems at random and only be able to install Windows and Office.