Some obvious ones:
- Allow fields to be exposed to the search engine, so if you add a string field you can optionally index and find tasks by searching for that string.
- Allow fields to be exposed to the list view, so if you add a field it can appear in the primary lists.
These are pretty straightforward. Sooner or later we're going to have to figure out what to do about aggregations, dashboard integrations, etc., but we can cross that bridge when we come to it.