This is a very minor point, but would go a long way to actually help adoption from Jira-expatriates such as me. Subtasks are currently viewed inline with other tasks on both the home page and the Maniphest page, like so:
<subtask>
<task>
<task>
<subtask>
<task>
However having the option of grouping subtasks along with their parent like this would be of benefit:
<task>
<task>
<subtask>
<subsubtask>
<subtask>
<task>
This need not be a global change, merely an option that can be enabled.
P1094 has a five minute hack which adds a very, very rough graph view field when dropped into phabricator/src/extensions/