I calculate time variances in some of my KPI dashboards that result in negative time values (e.g. if a procedure starts early it will be a negative variance from the planned time). Extending the custom number formats to allow for things like this would be great: "h:mm:ss;(h:mm:ss)". I currently have to color encode the values to be able to tell the difference between positive and negative values.
It would also be great if this format worked like it does in Excel: "[h]:mm:ss". The brackets allow the hours to extend beyond a base 24 hour period. Currently 25 hours renders as "1:00:00". With my proposed format it would render as "25:00:00".
[A related feature request is to support an actual TIME data type, that idea is at http://community.tableau.com/ideas/3315.]
[Edited 20150810 to add the TIME data type reference - Jonathan Drummey]