Which performance counters can you look at on a SQL Server Instance to determine it's performance and over all health? I am a student from Fontys University in Eindhoven, and I am currently carrying out a series of interviews to help with the development of a SQL Server tool and I would like to get feedback from the experts in the field. 

One of my questions is:

Which performance counters can you look at on a SQL Server Instance to determine it's performance and overall health? 

Particularly I am interested in the threshold values when good becomes bad.

Jamil Young 
Eindhoven
The Netherlands
Here's my Perfmon tutorial for SQL Server: http://www.brentozar.com/archive/2006/12/dba-101-using-perfmon-for-sql-performance-tuning/

For more counters and thresholds, here's a poster we did when I was at Quest: http://www.quest.com/documents/landing.aspx?id=11635