Although img@srcset supports the resolution matching use case in a limited way, it does not fully support the art direction use case. To support the the art direction use case, there needs to be some mechanism in place to force the browser to display a given image when condition is met. This is akin to the use case for CSS's !important rule.
The img@srcset spec lacks a script-based means to determine what resource the image element is displaying (if any). The use case for this is for testing and generally for a developer to know what is going on.
The img@srcset spec currently lacks a way for alternative image formats to be supported. This means that content negotiation must be done on either on the server, which is knows to be both problematic and does not scale well. Or it must be done on the client using a whole bunch of nasty hacks (e.g., downloading a 1x1 pixel image, seeing if it's supported, etc.).
The img@srcset solution lacks a means to programmatically interface with image resources, as well as access relevant attributes and methods that make the solution practical to work with (i.e., it shouldn't require complicated Regex or nested loops to manipulate values).
TODO: add link to Rick's example.
The img@srcset does not afford developers the ability to define the breakpoints for images as either minimum values (mobile first) or maximum values (desktop first) to match the media queries used in their design.
See: "secrete src" http://adactio.com/journal/5474/