How do I retrieve all forums?
ok, I got it to work, I think .

Accomplishing the goal requires two steps. First, find the "taxonomy vocabulary" for forums.  Then query the tree for that "taxonomy vocabulary".  

Step 1: 

curl -i -X GET \
 -H Cookie:SESSe6a353351dfbaaadbeef=B0ZkiW3-TNZ34dfe-uywhw3-UIWJJww \
 'http://myserver/rest/taxonomy_vocabulary?parameters\[machine_name\]=forums' 


Result: 

[{
  "vid": "1",
  "name": "Forums",
  "machine_name": "forums",
  "description": "Forum navigation vocabulary",
  "hierarchy": "0",
  "module": "forum",
  "weight": "-10",
  "uri": "http://myserver/rest/taxonomy_vocabulary/1"
}]


Step 2: Using that vid, Query the tree (via POST!!) 

curl -i -X POST \
 -H Cookie:SESSe6a353351dfbaaadbeef=B0ZkiW3-TNZ34dfe-uywhw3-UIWJJww \
 -H Accept:application/json \
 -H content-type:application/json \
 'http://myserver/rest/taxonomy_vocabulary/getTree' \
 -d '{ "vid" :"1" }'


Result: 

[{
  "tid": "1",
  "vid": "1",
  "name": "General discussion",
  "description": "",
  "format": null,
  "weight": "0",
  "uuid": "dbf914e7-42c2-45f6-b77a-e66a0da72310",
  "depth": 0,
  "parents": ["0"]
}, {
  "tid": "4",
  "vid": "1",
  "name": "Security and Privacy Issues",
  "description": "",
  "format": null,
  "weight": "1",
  "uuid": "7496bfd7-2cb8-4f87-a1e4-f45b1956a01e",
  "depth": 0,
  "parents": ["0"]
}, {
  "tid": "5",
  "vid": "1",
  "name": "etc....",
  "description": null,
  "format": null,
  "weight": "2",
  "uuid": "9fde5519-16b2-453d-852b-a8c416189e30",
  "depth": 0,
  "parents": ["0"]
}, ...]


Or, you can do a parameterized GET: 

curl -i -X GET \
 -H Cookie:SESSe6a353351dfbaaadbeef=B0ZkiW3-TNZ34dfe-uywhw3-UIWJJww \
 -H Accept:application/json \
 'http://myserver/rest/taxonomy_term?parameters\[vid\]=1'


The same data is returned.

My next challenge will be to retrieve the list of posts for each particular forum. 

Editorial comment: I saw this documented NO WHERE. I really don't understand how developers who use Drupal manage to use the Services module.