I like the `index` page, where I can see a list of tasks based on milestones or using other filters, which upon team review is indeed very handy.
I like how it has a way to interact with Hipchat/Slack, so that important changes made are ported to our chatroom as a kind of notification.
I like that it provides themes for color-blind people (not sure it's the case now, it used to be).
It provides too many things out of the box and doesn't let us filter out what we want to use. Feature toggling is direly needed but it doesn't seem to be the case on their development roadmap. We ended up using a very bloated interface containing a bazillion features which we have never used and will never. It's just a nightmare UX.
We use it to manage development work for our technology products. It provides an interface to doing product management which we find `compatible` to our workflow apart from tools like Github or Trello or Asana.