Dynamic Content Type? (field appears based on previous taxonomy choice) I want to show/hide a field based on a taxonomy choice (in the content admin side of things). 

Though having this field attached to every item of this content type does add overhead; I suspect it contributes to the easiest solution for a dynamic field.

I like the idea of dynamic fields, where only certain instances of an entity would have/need them, however, this seems like it could be quite an undertaking. (or my Drupal imagination is lacking today)
Try out the Conditional Fields module.

Conditional Fields for Drupal 7 is an user interface to the new States API, plus the ability to modify fields appearance and behavior on certain conditions when viewing content.
Conditional Fields allows you to manage sets of dependencies between fields. When a field is “dependent”, it will only be available for editing and displayed if the state of the “dependee” field matches the right condition.