No, that's not it.
Forget that even the most glowing reviews of TOR suggest this is WoW with a different theme - that's not the real issue (though it doesn't help).
Here's a comparison. I loved Diablo (1) - even though it was a hack'n'slasher, I loved the atmosphere. I eagerly bought D2, played it a bit, killed a mini-boss called Bloodraven. Shut down for dinner.
Came back the next day, fired up…and Bloodraven is there again! Everything "resets". This instantly and irrevocably killed the "immersion" for me. I hate that word but, for me, this rips down the curtain and reveals the game mechanics - it's no longer a "world", it's a click-to-farm excercise disguised as a game.
I'm not very bothered by most things but this one item kills a game for me - it destroys the idea that I am the hero and removes any idea of the world reacting to my actions. Even in the most unreactive game, a dead boss is usually a dead boss.
This is inherent in all MMOs. No matter how advanced, I can see the other players - it bothers me. They bother me. I know the bosses respawn. It bothers me. I know nothing can really change because all the other players need to access the content as well. It bothers me.
Is this just a neurosis of mine? Sure! I read comments from other people about how this or that breaks their immersion (no weather, bad animations, the lack of farms to feed the theoretical population, bugs that just don't bother me - a whole slew of things) - in general, these don't bother me. A lack of permanence and other players in my playground kills it for me. It just does.