json=true to specify the content type. Ideally, this should be from the accept header, but at the very least it should be possible to only specify one content type. Right now, I can specify json=true&xml=true.
Specifying content type as a parameter this makes debugging so much easier that I think it is worth it.
I agree with your other points. Specifically, the output type should be done the same way Solr does it: http://wiki.apache.org/solr/CoreQueryParameters#wt