The (experimental) JIRA REST api for issue properties has operations to:

• list all property names
• get the value of a single property
• delete a single property
• update/set a single property

What I really miss is the possibility to return all properties including their values in one single call in order to avoid the overhead of many single calls.
This possibility could also be resolved by adding another expand option "properties" to the search api via jql which will list all the properties including their values.