I like the idea of having a standardized way of dealing with actions (I run into this quite often when dealing with resources that are state machines), but I'm flabbergasted as to why something like a 'sum' action must be sent with a POST verb.

POST is supposed to be "used to request that the origin server accept the entity enclosed in the request as a new subordinate of the resource identified by the Request-URI in the Request-Line.", which is nonsensical for querying information like sums from collections, and furthermore is supposed to be non-idempotent, which isn't true for a sum operation.

Ideally "action" would be seperated into two different methods, one for manipulating resources (like state machine transitions, or other processing), and one (utilizing GET) for aggregations and computations on collections or resources.