The series ends with Voyager destroying a Borg transwarp hub, apparently robbing them of their ability to access a large chunk of the galaxy. But I seem to remember an incident a season or two earlier in which Voyager steals a "transwarp coil" from a Borg sphere, and are able to use it to go wherever they please. Since they aren't able to use it to get home, I can accept that it's probably a fair bit slower than travel via the hub, but I do recall it taking several years off their journey, suggesting that even without a hub a Borg ship should be able to travel from the heart of Borg space to the Federation in a few weeks. So why is the hub such a big deal?