SQL 2008 Resource Governor
I came across this post in Andrew Fryer's blog, which has a short demo of the SQL Server 2008 resource governor in action. For me, this will be one of the main drivers for moving to SQL Server 2008. Most of the issues I come across with unresponsive SQL Server instances are caused by run-away processes or bad application code consuming all the system resources. The ability to throttle these processes and gaurantee service for other applications using the instance is very compelling and could slash the amount of support calls significantly !
I attended a resource Governor session at Tech Ed IT Forum in Barcelona last year and gave a live demo of the features at my Christmas team meeting, but I can't wait to really get my hands on this feature and give it some thorough testing.