I would say Networking, Servers and Security.
Networking: Knowing how LAN/WANs work and having the ability to Install, Configure and Troubleshoot Routers and Switches. A Cisco Certification like a CCNA (Cisco Certified Network Associate) is good to gave.
Servers: Knowing how to Set Up, Manage, Configure, Troubleshoot, Upgrade Servers. This is a very broad thing so which "type" of servers depends on the company. But a standard Microsoft Cert like MCSEA/MCSE or MCTS would be good to have.
Security: Security is a big concern so knowing how to secure your network, configure Firewalls, Monitor Network Traffic, IDS/IPS, etc... There are many certs for this, good one to have could be CEH (Certified Ethical Hacker).
Basically to be a System Administrator you have to somewhat be a "Jack of All Trades".