Your SLA must describe specifically the service that is being offered.
Key points with SLA’s is that they are:
a) supportive of the customer requirements
b) achievable
c) measurable
It must be possible to define KPIs within your SLAs that you can measure to drive performance. It is imperative that by improving these KPIs you are improving the experience for the customer – this is the most difficult part as customer experience is pretty much a perception of service rather than cold hard facts that you can measure. You need to really understand how the customer uses the service to get good alignment of KPIs to customer experience.
Look for alignment of Service Hours to intended usage times. Recovery times in line with a business impact assessment of loss of service.
An SLA could combine factual KPIs (number of widgets processed, availability, time to recover etc) with a perception based customer rating, and your reporting would (hopefully) demonstrate a link between the two.