I like that I can break down issues into sub-issues.  However, on an issue that is a sub-issue, I cannot add sub-issues to it.  I would like to do this because I'm using issues to create estimates, and I would like developers to break down issues into chunks that can be estimated with less than 8 hours of work.

It would be nice to have the ability to create sub-issues of sub-issues ad infinitum.  Then, have the estimates for the sub-issues total for the parent issues.  i.e. If I have a three sub-issues with an estimate of 3 hours each, the parent issue would automatically get an estimate of 9 hours.

On a related note, if I could create custom link types for linked issues this might help.