The way you described your fandom of KotOR but hesitance for TOR pretty much exactly sums up how I feel as well. I also have never played an MMO and $15 bucks a month for a very casual gamer like myself seems to be a waste of money.
Unfortunately, not having experience like you in MMOs, I can't answer your question on how they are planning to incorporate this "epic storyline". However, they've made such a big deal of it (i.e. calling it a "pillar", giving stats on how much story is involved like 5 novels-worth of writing, etc.) that I can't help but believe they really will accomplish what other MMOs have not been able to do. Plus, like you said, it's BioWare so I tend to give them the benefit of the doubt.