Years old issue at this point, but my need for such functionality hasn't changed in a decade.
I guess this makes GitHub, GitLab and Gitorious better than BitBucket since you can use this functionality through markdown. I'll have to move my projects over to a paid Github account since BitBucket can't provide such simple and important tools for complex project tracking.
It makes sense to have sub-tasks integrated in bitbucket.
I found the need for it when I wanted to create a task that cannot be completed before a previous task, but have to way to tie the two tasks together.
Please reconsider (or is there another solution)?
This would make things a lot cleaner for me.
I think the benefit of moving to jira comes in its many other features, not specifically subtasks, so there would still be plenty of draw to upgrading if that's a concern.
The most simple design for this in my opinion is just one single additional line of text field, where you can enter issue numbers. These will then be visualized as referenced tasks.
Nevertheless, real subtasks would be much better. Please, reconcisder your decision.