Warning: This post is just my opinion, I'm not trying to start a flame war or anything.
With the big push of the Legacy update I think we can all agree one of the major goals BioWare is setting for SWTOR is that people will play multiple classes. This is great, and provides bonuses for playing other classes via the legacy rewards.
But, and this is a big but, there is just too much shared content not just between classes on the same side of the war but globally. I've played Sith Inquisitor and Jedi Knight to level 49/50 completing all the quests and planets including bonus series besides Heroics.
I want to play other classes, I even started a Trooper; but I get to Coruscant and other than the class story it's the same exact quests as Jedi Knight AND Inquisitor (of course with a different quest giver for the Imperial side). I'm flash forwarding in my mind and thinking about replaying all the quests and grind from Nar Shadaa, Belsavis, Taris, Voss, etc., and it's very disheartening to think about the boredom in redoing all those quests over again.
To me that's the biggest obstacle to replayability, I don't think anyone is going to want to do the same quests 4 (for quests shared among all classes of the same side) and or 8 (for quests shared among both sides) times. I know it's too late now to change without basically wiping all characters and starting over (which will hopefully never happen); but there definitely should have been more quests specific to each class. Even if that meant each class is much faster to level to 50 and has less content on an individual class basis.
I know many people will play all 4 or 8 classes and more power to the ones that do, but I don't think I'll make it past 2-3 classes without getting too bored of the shared content.