If I create a story, but don't assign an epic to it, then I create a few sub-tasks, then I assign an epic to only the story, the sub-tasks won't have the epic assigned.

Because the view of an epic with all of its stories and tasks only shows the ones with the epic assigned, and - here's the real problem - the estimates that roll up to the epic don't include the unaffiliated tasks.

(1) Ideally, when assigning an epic theme to a story, I have the option to cascade it to the children of that story.
(2) When adding new tasks to a story with an epic theme, it would be nice if they would auto-adopt the story's epic theme.