For administrators who do not use System Center Operations Manager to monitor their Exchange 2010 environment, a thorough understanding of the most important performance counters and their thresholds is critical to establishing a performance baseline and monitoring plan to proactively monitor your Exchange 2010 environment and troubleshoot and resolve issues when they arise.
A blog article is being written about this topic by the Exchange team, but in the meantime,an Excel spreadsheet has been released that has suggested counters and thresholds to monitor. This spreadsheet can be downloaded from here.