Performance counters are very important to determine the performance of the server. As an administrator/Consultant/architect you should aware of the basis performance counter which are important for both exchange and administrator prospective.

Microsoft TechNet article (http://technet.microsoft.com/en-us/library/dd335215) talks in-depth of the performance counters for each of the exchange role and we will touch base some of the important ones on server role basis

You also find the spreadsheet which talk about the Exchange 2010 performance and threshold counters  from the below location

http://gallery.technet.microsoft.com/Performance-and-Threshold-d32ff5a6