I am trying to understand the logic with the SharePoint client.svc (_api) Web service.
For some queries, for example List GetItems, or search queries, SharePoint requires the use of POST in ajax calls. Queries are just read operations, without server side updates, so why is that?
Using POST is heavy, as you need to retrieve a request digest token first. I'd rather use a simple GET.
For an example of query that requires POST, see the answer on this page: http://social.msdn.microsoft.com/Forums/sharepoint/en-US/92cccd65-ba4c-4870-a858-7cd0e38a0482/how-can-i-use-caml-queries-with-the-rest-api