First is speed. I've been able to get speeds of ~50MBps (not Mbits) on private trackers, granted this is dependent on Internet connection more than anything but I get 20-50% of that speed on public trackers.

Second is retention and breadth of selection. If you're trying to download the latest Marvel movie then every tracker is gonna have that, but if you're looking for an older movie then it's much harder to find on a public tracker. And if you do find one, it's likely to be seeded by 1 person and you can only squeeze 10KBps out of it.