Would "as much as the next guy" be a simile, or just an idiom? I am working on a lesson plan for similes, and was not 100% sure.
I argue it is not. The simple description of a simile is a figurative comparison using "like" or "as," however these words can be used for literal comparisons as well as (see what I did there?) figurative ones.
For example, one might say that a tall, narrow bottle appears larger than a short, wide bottle at first glance. But really each has a one quart capacity. The one holds as much as the other. This is clearly not a simile.
In parallel, a statement like "I appreciate theater as much as the next guy" is making a comparison between mine "the next guy's" capacity for enjoying the theater. While the next guy might be a figurative entity, the comparison between the measurements is not.
The ODO definition of simile is
1 a figure of speech involving the comparison of one thing with another thing of a different kind, used to make a description more emphatic or vivid [emphasis mine]
Here, 'the next guy' is hardly 'of a different kind' in any meaningful sense, and the 'used to make a description more emphatic or vivid' requirement is not fulfilled.