The whole idea behind REST resources, though, is that the HTTP method expresses the action, and the URI identifies the object acted upon. In that conceptual scheme, there is neither need nor place for verbs in the URI; POST /topics/add is redundant, because POSTing to a collection already has the semantic value of "add an item to this collection". Similarly, if you're modifying an existing item, you don't need to PUT /topics/23/edit or what-have-you; PUT already means "replace the existing content with the request body I'm sending", so all else that's required is to identify the particular content to replace, for which /topics/23 suffices. Having the HTTP method and the endpoint gives you all the clarity you need in terms of what a request does, and to which thing it does it.
An additional, and very non-trivial, benefit of this scheme is that it is standard and discoverable: "standard", in the sense that these basic HTTP operations are defined in RFC 2616 and exhaustively documented there; "discoverable", in the sense that knowing some endpoint represents a REST resource API instantly tells you quite a lot about how you can interact with it, and you can find out the rest from the responses you get when you make API requests -- which responses RFC 2616 also explains how to interpret.
Not everything is simple. REST resources are, and they need not have a complex API. Poorly thought-out APIs for simple things require a lot of documentation, that mostly being lousy explanations of the interactions between an incomplete set of edge and corner cases. Well thought-out APIs for simple things barely require any documentation at all.
The REST resource API is a sterling example of the latter sort; once you're familiar with RFC 2616, you can discover everything else you need from the responses you get to API requests. This is not so when redundant verbs -- POST /topics/add -- are used in a backwards namespace -- POST /add/topic -- with uncertain naming -- /add/topic, so if I want all topics do I GET /topics or /topic, or /get/topics, or what? What makes all this confusion more worthwhile than just using REST resource endpoints? Do you want people who consume your API to curse your name for making them deal with a bunch of needless impedance matching, or to effortlessly retrieve what you're offering and do something amazing with it?