These are some ideas that I got (directly and indirectly) from a Google Tech Talk (At Google's 2010 I/o Developer Conference).

:: Features ::
 - Create libraries to execute functions
      - E.g. JS library that is hosted on API server which gives developers easy 
        access to methods and is kept up to date.
      - PHP library that is compressed (zip,tar gz) and an email is sent out with 
        announcements on changes and versions.
      - If the library is in javascript then it can return an Array of Objects, numerically
        represented keys (for the elements).
      - Also, the api could return an object that holds the array of data along with 
        common functions for future calls.
        Service.videos.search(Object JSON_object, Bool return_object_with_functions);
        Service.videos.search({
             "query": "funny cats",
             "count": "20"     
        }, true);

        {
              "data": [
                   {
                       "title": "",
                       "author": "",
                       "views": "",
                       "likes": ""
                   },

                   {
                       "title": "",
                       "author": "",
                       "views": "",
                       "likes": ""
                   }
              ],

              "next": function () {
                    return Service.videos.search({
                        "query": "funny cats",
                        "count": "20",
                        "start_id": "s50h86kq"
                    }, true);
              }
        }       

      - Service.users.update( auth_token, {
                "email": "user@domain.tld",
                "bio": "I like cats."
        });
      - Service.videos.search({
                "query": "funny dog",
                "count": "10", 
        });

 - Partial GET's and PUT's
      - http://api.domain.tld/users/edit/?email=user@domain.tld&bio=I+like+cats
      - http://api.domain.tld/users/?username=adam&fields=name,age,url,bio&format=json
     - Service.videos.search(Object [JSON] options, Bool return_commonly_used_functions, Array [JSON] fields);
      - Service.videos.search({
             "query": "funny kids"
        }, false, [
              "title",
              "id"
        ]); 

 - Custom Variables in requests
      - http://api.domain.tld/image/edit/?image_id=2hg2h3g2&method=rotate&degrees=90

 - "Templates" for response formats
      - Just like a regulat HTML template, you use a template for response formats (XML,JSON,Atom)

 - Versioned API's
     - http://api.domain.tld/v1/user/edit/
     - http://api.domain.tld/dev/user/edit
     - http://api.domain.tld/experimental/

-- 
Adam Shannon
 Web Developer
 http://ashannon.us