I would echo what some of the other posters have said in that an MMO is going to play differently than a single player game like Deus Ex or Mass Effect. In a single player game, especially one of the better story-driven ones, you are immersed in a very specific storyline, and you play it through to the end -- in a good version of that kind of game, like the ME series was (despite the ending brouhaha), your choices influence outcomes and it feels very satisfying from the narrative point of view. But you're still playing as Commander Shepard -- it may be your take on Commander Shepard, but the story is really Commander Shepard's story. And that's cool -- it's what those games are about. An MMO, however, is much more about creating your own story with your own characters. Even though the game provides missions and quests and so on, the character is yours to develop as you wish, to approach things as you wish in a wide-open manner. You're more like a sculptor than you are in most SP games.
I played STO when it was released a few years ago for several months and then quit. I just recently returned because my son had decided to pick it up and play it, so I figured I would give it another look (and play together with him a bit). I can say that it is MUCH better than it was when it was released, and in so many ways that it hard to list them all. I am very much enjoying playing through the storyline of the Romulans (always been a Romulan fan), and I can't say if/how the Fed side has been improved, but overall the game is quite satisfying to play now and in a memorable way. I have played alot of MMOs in the past, and I'm quite pleasantly surprised to say that STO, in its current iteration, compares favorably with other MMOs, and quite so.
Actually I believe that some day one of these game companies are going to spark a brain cell and realize that they could make a single person game using the MMO type cloud of resources that is open ended and continuously supportable and upgradable through that cloud.