What are the top 3 performance Issues that you encounter with your SQL Servers?
Off the top of my head - top 3 query problems:


Implicit conversions
Bad indexing strategies (too many or not enough or the wrong kind)
Using SELECT * instead of naming just the fields you need


There's a lot more around server-level config problems, database schema problems, hardware problems, etc.  I wrote a script to quickly analyze servers looking for these kinds of problems:

http://www.brentozar.com/blitz/