What you monitor, and to what extent, depends on your specific deployment. In general, however, include the following elements in your monitoring strategy:
Server activity such as resource usage, server status, and connection information
Database activity such as cache, transactions, locks, and log information
Disk status including available disk space and threshold information
Replication activity including status (whether or not replication is running), and the state of synchronization
Indexing efficiency including unindexed searches, search filters, and frequently used indexes
Security status including failed bind attempts, open connections, and effective rights