Backlog helps us track progress from granular updates to larger-scale program reworking. The ability to integrate with Slack is also super helpful.
Email notifications are effective and clear. Icon personalization is a nice touch.
I liked the inclusion of the Wiki as well, and I like the tiered subscription model they use, making it easy to find a level of feature availability that suites your team's needs.
I think Backlog shoud list parent issue and issue has no parent issue. About issue has parent issue, you can hide and when user click on the arrow, it can be showed.
The only thing that I would say that's negative is that it runs slow at times but I believe that's a result of our environment.
Nothing comes to mind at the moment as something to dislike.