I'm a web developer, and I charge my clients "hosting and maintenance" fees. This covers my raw hosting costs, but it also includes overhead for server maintenance and handling the occasional "critical bugfix". That's usually good enough for my clients.
I now have a client that's looking for a service-level guarantee. I've never done an SLA, but I figured I could offer "99% availability for mission-critical features" and in the case of a failure give a "prorated refund on hosting for time duration of lost availability". It's a relatively simple application, so I'm not expecting to have to troubleshoot very often.
Is there anything I'm not thinking of that would make this type of agreement a bad idea?