SW:ToR quite frankly is a really good game.......a really good SINGLE PLAYER game. The story arcs are good, most of the stuff is well drawn out. But there is no endgame once your done with the PvE side of it. . No large scale Open PvP that tens of thousands of players wanted and by bottlenecking all the PvP'rs into lame arena style instances, players are done with it and leaving ToR in droves.
This is the main problem, as I see it, with just about all MMOs out there. Once you hit the endgame you can...?
The standard MMO model gives endgame players raids and small-scale PVP. The raids are fun, but ultimately they're just a hard PVE quest that you do over and over and over. Once I've done one a couple of times, the only thing that gets me to repeat is the social aspect of raiding with friends. Likewise, PVP is fun for a while, but I eventually get turned off by how little it matters.
In my personal opinion, meaningful PVP is the best thing that a MMO can offer, both in the endgame and before. "Meaningful" describing PVP that actually affects things. In Eve Online, for instance, there is a core of space controlled by NPC factions surrounded by a gigantic ring of uncontrolled space (which is also where most of the wealth in the game is). Everything that happens out in this ring of space happens because of what the players do. They form their own alliances and lay claim to space and develop it, or they invade someone else's space and try to take it, or any number of things. The economy in Eve also follows a PVP model - everything better than the very basic gear is made by players out of materials mined by players and the market is entirely player-driven - but that's really just an extension of the meaningful PVP principle.
Most MMOs don't offer meaningful PVP mainly because they use a sharded server model - the LOTRO Landroval server has to match the experience of the Nimrodel server, so you can't allow folks on Landroval to clear out Moria, for instance. STO notably doesn't follow this model. Certainly there are instances, which prevent truly large-scale PVP, but that could be worked around. Likewise, the ratio between Fed and Klink players could also be dealt with:
Imagine something like a no-mans land between Klingon and Federation space. The "standard" PVE missions wouldn't take anyone here, but there would be PVP missions there. Imagine if the result of these PVP missions determined which faction controls which parts of this space. Something like the population of active Fed players in this space could be compared to the population of active Klink players in this space to scale things. You'd want to give some sort of benefit to controlling space, too, which would probably be the harder thing to do.