Brief history on why WoW was successful and what will/can make SW:TOR successful
Ok first, let's get this clear, there are no game killers. WoW never killed EQ, it simply became a more successful game than EQ, EQ is still up and running today, so saying an mmo will kill another mmo is stupid and this is not about killing wow. This is mainly for people who are saying SW:TOR is wow in space, and will fail.
First off let me say why WoW succeeded
First off, the big mmo at the time of WoW's launch (WoW was not the first mmo to ever exist, and neither was Everquest shocking I know) was Everquest, and at that time there was a lot more of WoW is just a casual game, and people didnt think it would be that successful. (heck even blizz members said they would be happy with around 250k subscribers)
Why did WoW overcome Everquest in the amount of subscribers? And what made it more successful?
It did not revolutionize things, it simply improved on them. WoW had everything Everquest had in it (at its launch, not now, at its launch), WoW launched with no Battlegrounds. WoW had questing, Everquest has (some) questing. WoW had raiding, Everquest had raiding. Etc.
What did WoW change, or improve?
First off it improved the levelling feature it made it faster (much faster than everquest), and it added questing to make it 'more fun' and 'less grindy'.
Secondly, Everquest did not have instanced raids or dungeons. WoW changed that to have instanced dungeons and raids.
Thirdly, WoW's class system was not modelled after Everquests system but it made its own. Mainly people would ask, 'what class is closest to this class' and the majority of time it would be, you really can't compare those classes, but this class is the closest.
Fourthly, it was in a popular universe and setting, which let you explore and go to 'familar' or 'reconizable' places that you saw in the Warcraft series, believe it or not this is a big deal.
WoW also made other improvements like this, and also removed a couple things that helped make the game better.
Basically it did not try to change or make a completely different MMO system, it took the current MMO system and made it better. Also the improvements they made were noticable right when you were level one, and you didnt have to get to the level cap to notice the improvements, however you may notice a couple more improvements a long the way, it was noticabally better at the start.
Afterwords however, during expansions and patches, blizzard added new features to WoW to make it better that changed the MMO system. (like battlegrounds, arenas, etc.)
What made other MMO's not be as successful as WoW?
They tried to do one, or a combination of the following.
They simply took everything WoW was and put it in a different setting. Nothing new, nothing improved.
They tried to add new features while not improving the old ones. (which games like WoW was doing) While these new features may be good, old ones that need improving are still there.
The improvements were not noticable until endgame. This is a big one, because a good example is Rift, they did excellent with the end game, however their levelling system was flawed and basically a vanilla WoW clone. This made people who play it get bored and say 'I should just go play WoW, it's the same thing' even though the endgame may be that much more improved.
They try to revolutionize the mmo, basically they try to make is completely and drastically different where they create their own new MMO system. This hasnt been done much, but the majority of the time this doesn't work becasue the entire new MMO system may be flawed.
What can make SW:TOR succeed
First, it is improving on the current biggest MMO, WoW. (which is why some people say it is similar)
Secondly, The improvements are noticed from the very beginning, by making levelling more fun and actually enjoyable, even for an MMO or WoW veteran.
Thirdly, it is placed in a familar setting and universe.
Fourthly, the classes are different and not comparable to WoW's.
Fifthly, it isn't trying to add tons of new features to make itself stand out from WoW, it is simply improving WoW to make it stand out from WoW.
NOTE: This does not mean SW:TOR will be a so called 'wow killer' that will never happen, but it does mean it can be very successful and possibly give WoW competition.