Progress!
The content element has to be defined as content_raw. Still no luck with categories though.
{
"title": "ldap-service 1.0.3 dev deploy",
"status": "publish",
"type": "post",
"content_raw": "Version 1.0.3 deployed to Dev servers by $user",
"terms": {
"category": [
{
"name": "Dev",
"parent": {
"name": "Releases"
}
}
]
}
}