One thing that the circle metaphor does add is nested groups, which I can see being important for a different set of use cases.
That said, I tend to agree. Trello works at least in part because it's immediately intuitive to anyone who's ever made a "to-do" list or used cards-on-a-whiteboard to manage a project.
The Droplist approach is a cool idea though, and for some things it might be better.
This is true - the group (used to categorize task circles) grows to fit the content. We did play with allowing task circles to be resized to show priority/effort, etc, but couldn't get it to work well. We may revisit it, though.