SWTOR's biggest flaw is that it is half-heartedly trying to be another KOTOR game - Sith, Jedi, the Force, companions, and high adventure - but it can't be, because it is an MMO. It is bound to certain things, like there has to be a Republic, and there has to be a Sith Empire. A single player game would not be as bound to those constraints. Imagine a KOTOR 3 where you found the True Sith (Vitiate's Sith) and the Republic well and truly fell as a result. It was already at the brink of collapse in KOTOR 2. Ushering in a long period of darkness and tyranny, where the Jedi have to claw their way back, piece by piece. SWTOR can't do that because it has to keep the Republic together and give Republic characters things to do.
The previous expansions always raised an eyebrow with me because it felt like nothing you did had any point to it. I knew the Eternal Empire just didn't fit the universe they had set up. So I was glad to see Onslaught reignited the conflict between Republic and Sith again. I played my trusty Sith Juggernaut, Darth Vilrost, who's always been a Sith loyalist, and I had a good time. I was glad to be able to make choices to bring the Eternal Alliance to an end and get back to where things were. I liked seeing old characters again, and killing companions who betrayed me. I think I've killed most of the original companion list at this point - except for the companions who stayed loyal. I thought the writing was pretty good, certainly the best since the original class stories. I get the feeling Drew Karpyshyn and Alexander Freed were more involved this time around.
The game is still fatally flawed from a story point of view, for the reasons I stated earlier. Gameplay is really nothing to shout about. I don't do flashpoints, operations, PVP or any other endgame content in SWTOR because the combat is horrible. I suffer through it in the story but I don't think I could face it in multiplayer. So I play the single player content, unsubscribe, and resub when there's new single player content. Yep, not a good MMO.
My personal wishlist for SWTOR would be to have new class stories as good as the original ones, with all the companions, but that just isn't going to happen. I thought this expansion ended on a good note, so I'm happy to play more, but if this expansion had been like the ones before I would have taken a different view.