[pirateking]

I consider visual design to be a part of interaction design, and taking one without the other is a failure to understand design (to the limited extent it can be understood at all). An example: a traffic light's visual design is fundamental to its interaction design.

[seanmcdirmid]

Right, but visual and interaction design are different activities with different outputs. An interaction design language is more about patterns of flows while visual design language is about style and metaphor. Contort them at your own peril.

[notduncansmith]

Style and metaphor are integral parts of interaction design. When a user clicks a button to dismiss a dialog, what should happen?

If the experience you're trying to create (in line with the brand) is more playful, then maybe the modal should bounce out of view. If it's a very serious corporate app that people are just using because they have to, then the modal should either quickly fade, or just immediately disappear. One would be inclined to call this "style".

What about when a user is asked to input a date and time? There are two visual metaphors that come to mind: a calendar and a clock. The interaction designer has a few choices here. Date + number picker, date + clock (cool metaphor for selecting a time, but a non-standard UI pattern that could confuse people). This is an interaction design problem, not a visual design problem.

Perhaps you're referring to style and metaphor as the color of buttons and the background texture of panels. That I would say generally falls more into the realm of visual design; however, colors of buttons convey different semantic information, which affects how people interact with your app.

[pirateking]

I naturally see the visual language as an output and the interaction language as an input, both interfacing with the holistic design space that I am trying to explore across the design process. They are not two discrete activities, but two forms of participation in a singular activity, with a design of the thing as a single output. I have taken this approach to designing things for a long time now, with no great peril beyond philosophical curiosities such as this.