It's unfair to compare the success of a game based only off it direct competition. People will usually say something is a WOW killer or Halo killer comparing an upcoming game to others in the same genre, but it's not an accurate example. For something to really be a direct threat to an existing IP, it has to have the time to develop and grow, not run from wild speculation. World of Warcraft is a giant game because they make it fun and easy for anyone to get into and offers large worlds for exploration.
Ideally the same can be said for The Old Republic.
While TOR is already being hailed as a giant of a game, and the best MMO ever, no one really knows how the masses will respond to it. LucasArts has a habit of watering down their good content, so even if TOR starts off great, there's no telling what the future will hold. The only real thing we can base any questions on TOR about it BioWare's recent work(s).
Dragon Age:Origins, and tomorrows Mass Effect 2.
Both games will be primary story based, so look for an interesting story with TOR. Both also establish a group of players, so I wouldn't be surprised if we found another lovable droid (HK-47) in TOR. But the real clicher is going to be in the future support. ATM Mass Effect is having a lot of DLC, and honestly that's something that TOR is gonna need as well.
MMO's can't ever survive on just what it launches with, eventually people move on despite how fun it is. For The Old Republic to really win big, it's gonna have to provide something for the hardcore to do, because there will always be people like me that run through the content as fast as they can because they enjoy it, and want more.