Short version : An RDF Affordance is a resource description which gives a client all the information it needs to perform an action.
My last post about what a Data Web Browser might look like led to some fertile discussion on G+. Essentially Mike Amundsen neatly reframed the question to being one about affordances, pointing to a bit of related prior work by him on Hypermedia Types.
We hold this truth to be self-evident, that presented with a simple application scenario a Web Architect will abstract it into a form that will take decades to implement.
Web Intents and Actions
Tantek has spotted the potential around intents and in Web Actions: Identifying A New Building Block For The Web looks at common features across existing systems like Blog this, Digg, Read later, Follow, Like, Share, Tweet, +1 (he uses "Actions" instead of "Intents" for essentially the same idea).
We hold this truth to be self-evident, that presented with the potential for open-ended innovation a Microformats Geek will start paving cowpaths.
On the Wiki - RdfAffordances - Mike has brought the abstraction back down to ground with some more detail of RDF-oriented actions, and with a view to hacking an implementation (on my virgin node.js installation) I've started a vocabulary - AffordanceVocabulary - this may change fairly soon, apparently Michael Hausenblas has done a vocab in this area, that'll get precedence if there's overlap/conflict.
We hold this truth to be self-evident, that offered a simple application scenario a Semantic Web Geek will always create a vocabulary that obscures the purpose of the application and that no-one will ever use.
Not entirely joking...
There is one high-level abstraction I've noted on that vocab page that is probably useful. There's a natural boundary between affordances that are essentially just HTTP (e.g. click through link, replace a page) and those which require more complex interations. For now at least I'm calling the former Actions (let me know if there's a better word that doesn't clash with Tantek's usage) - they are around the scope of Mike's Hypermedia Types and the latter Intents - around the scope of Web Intents.