Imagine two level-capped troopers of the republic sitting in a cantina in Coruscant and exchanging life stories: They would find that their life stories are to a very large degree identical; they visited the same planets in nearly the same order, they got betrayed by the same people, they had the same companions, and probably even the same romantic partners. Their life only differs slightly in what bonus series of quests they skipped, and in what decisions they took during dialogues, although they will find that these different decisions did not lead to them having different lives.
Star Wars: The Old Republic is an extremely linear game, even more so than previous level-based games like World of Warcraft. Not only aren't there enough different planets at the same level range to really allow a choice of visiting one or the other (which could be fixed over time), but the class quest line doesn't allow much variation. That linearity has two consequences: It lowers replayability, and it makes the player less engaged with the story. You aren't playing through *your* story in SWTOR, you are playing through the fixed story of your class.
While linearity of storytelling is very much "in" at the moment, people are beginning to see the negative effects. The high-level zones in Cataclysm are very linear, and Blizzard is starting to call that a mistake. Edward Castronova not only says that linearity causes SWTOR to be "dead", but also cites a counter-example of a virtual world which seems much more living, for being not linear: Skyrim.
I think many people would be delighted if there was a MMORPG which had a structure similar to Skyrim, where you can go wherever you want, and find bits of story everywhere. Your overall life story might still consist of canned bits, but the order of those bits is probably very different between any two players. And because there is no linear main story, the actions of the player can actually change his personal story. In Skyrim, for some players their companion Lydia died, while in SWTOR you can't kill your companions even if you want to.
What tends to get into the way is the current structure of MMORPGs as games of character advancement. Your character today is stronger than yesterday, and that changes what is appropriate content for him. Skyrim gets around that problem by cheating: To some degree the mobs simply gain levels in parallel with you, so it doesn't really matter at which level you discover that forgotten cave. A MMORPG could do something similar, but only with instancing. Or we would need systems where characters don't get stronger, but instead develop horizontally, getting more choices of abilities over time, but not necessarily stronger ones.
What would be the purpose of all that? Both from the point of view of the players and the point of view of the game company, the question is how to increase the longevity of MMORPGs. That depends on giving the players something to do, a living world to inhabit, which inherently lasts longer than a story with a fixed beginning and end. As quoted from Bartle yesterday, the goal is to have virtual worlds as places you like to return to, not just because you have a purpose there. It would increase the appeal of virtual worlds as social networks, if these virtual worlds were places you could explore and experience at your leisure, and with your friends, without quest lines and levels getting into the way. Games like Skyrim show that this is possible, and that these open worlds can sell extremely well. It is just a matter of time until somebody realizes that this could be a way to make a MMORPG which really differentiates itself from World of Warcraft without being less successful. Unless Blizzard gets there first with Titan. But I think the concept of the linear MMORPG has been tried and found wanting. It is time for more open structures, with individual stories that engage the players more, and worlds you would want to live in.