I've mentioned 'ws-reputation' before here.
Moving on from the whether or not I can use the services I desire - and use them productively at that - I'm still concerned that the service I've chosen may simply disappear, or change annoyingly. I don't mind so much about this when I'm adding controls to my blog or other personal stuff, but if I was developing professionally for clients, this could be a real problem.
So, when I harness a service as part of a wide application, I'd probably like some kind of metadata returned (or described in some other way) that forms a 'contract' - in terms of quality of service - from the service provider.
Consider that if I knew:
Then I could build code at my side to cope with maintenance (I know that I can cope with an error in any case, but maintenance information may allow me to schedule my own downtime, or a different kind of degradation in service). I'd also be aware of upcoming API changes, and essentially the reliability of the service that I'm tying my paying customer to...
This 'quality of service' could automatically be monitored for reliability - so irregulare maintenance, changes and so on could be tracked against an API, which would effectively form a 'reputation' for the service. Being able to choose against that simple measurement (let's say 1-5!) would be a powerful differentiator in an ocean of APIs.