I particularly liked that there are very nice cli tools for it: https://www.pivotaltracker.com/integrations/command-line
FYI a dozen people would be a huge team at Pivotal. 30 folks working in a single backlog is unheard of. But I hear Pivotal became a different place after Rob left, so I don't know what it's like now.
For me, the most impressive thing is that Tracker manages to be simple enough but not simplistic.
I've used a lot of tracker products, and I don't consider Pivotal Tracker to be particularly opinionated. That might just mean it works the way I prefer of course! But we do not do dogmatic methodologies, and Tracker is flexible enough to add zero friction to our process.
What I learned from working at Pivotal (with @stickfigure actually) is that there is the way that Pivotal uses it and the way everyone else uses it.
I think the key is to have good search, which Jira does not remotely have.