The Old Republic has an interesting mix of zones. There are zones unique to each faction, like the starting planets and the capital planet. There are "shared" planets, like Hoth, where both sides quest (and gank, if on a PvP server) together.
And then there is Taris.
Taris is a city planet, destroyed by planetary bombardment by Darth Malak back in Knights of the Old Republic. It's now infested by rakghouls, a vicious bunch of monsters who infect their victims and turn them into more rakghouls.
Taris is a level 16-20 zone for Republic and a level 30-36 zone for Empire. But Taris is not a shared world!
Both zones use the same geography, and the same planet, and the same concern with the rakghouls. But on the Republic side, the Republic holds Taris and is attempting to reclaim it from rakghouls and pirates. On the Empire side, the Empire holds Taris and chasing the remnants of the Republic off it.
It's as if the two Tarises exist side-by-side in parallel universes. In one universe the Republic is dominant, and in the other universe the Empire is ascendant.
It makes me wonder if SWTOR is a shared universe between the two factions. In World of Warcraft, it's obvious that the two factions share the same world. You can go and visit the home cities of the other side.
But SWTOR seems to be running two parallel universes that only occasionally overlap. Which is really odd at first, and seems quite unusual. But there are advantages. With two separate universes, the storyline can unfold differently for each side. One side can "win" without the other side suffering a "loss".
Still, this quite caught me by surprise. I'm so used to the shared world, that other setups seem hard to grasp.