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Multiple paths through the story.
If there were, say, three different planets to choose from for any given level bracket (rather than one), and if your personal class story could branch into any of them depending on your choices, that would add enough freedom and replayability to keep me around for years. You could still lead them all back into a single zone to conclude the plot, but if there were distinctly different paths to reach that ending, it'd feel more like a story you're making, rather a story you're being told.
It's the overall linearity and lack of choice that drives me away from SWtOR. I love the look of the game, the way the classes play... I love a lot of the features like companions, personal ships, etc. But it is by far the most linear, on-rails MMORPG I've ever played, and that just ruins it.