Something has to give, even with the deep pockets that Electronic Arts and LucasArts offer, there'll be some kind of issue leading up to that fateful day. My hope, of course, is that Star Wars: The Old Republic's hiccup is minor, rather than major. I just don't think every feature of an MMO is immune to damage when a developer promises three or four games' worth of content in a single title.
Who knows, maybe there'll be more bugs than an average MMO launch. Perhaps the game will be intoxicatingly delightful to play, except when it freezes or crashes. People would put up with that, right? Well, no they wouldn't, actually. A minor amount of that and usually everything can be managed, but if a game has these kinds of issues the "word of mouth" buzz can be significantly diminished.
Perhaps PvP will suffer at launch? That was essentially the case with World of Warcraft -- well that, and there were the queues... oh there were queues. It'd be strange, though; a game pitting Jedi and Sith against each other that manages to under-deliver on PvP. Considering the emphasis being put on story content, it seems like PvP would be the place to look for issues. Although, since we know zilch about BioWare's plans for it, discussing its potential failings is quite literally impossible.
There's always the possibility that no one particular aspect of SWTOR will feel off. We could end up with a game that has passable combat, PvP and other basic MMO features. Is this the trade-off for more story than any other single videogame created? If so, it could be a worthwhile sacrifice for an MMO that allows me to reach maximum level almost solely through story-based content. Being able to experience a BioWare story while earning those glee inducing dings is something I could do for, well, probably every class in the game. That even seems to be what BioWare is hoping the majority of its players will do, too.
Personally? I think the most likely scenario is that BioWare delays the game before pushing it out in an incomplete -- so to speak -- state. Even with the shadow of Cataclysm looming, I get the feeling these people would rather do it right than wrong.