This excellent article is about user interface mechanisms and their physical metaphors, a pretty small subset of interaction design, despite the author's attempt to define the term.

More broadly, interaction design inhabits the elusive void that occurs between people, products, services and environments.

Once you're down to deciding between one interface mechanism or another you've abstracted the bigger, more important picture relating to what and why the interaction is taking place, or even if it should.

Just a reminder to zoom out every now and then.