My wife and I were discussing the problem of segmentation today - yes, we're nerds and both work in academia - and came to a solid conclusion: the problem with MMOs is that they struggle to appropriately match players and content difficulty.
In single player games, you generally choose your difficulty (Easy -> Nightmarish) with no consequence to your ability to see the story, gain powers, level up and generally enjoy the game. There's no social stigma attached to playing the game on "Easy", and you can always go through and bump it up after a few play throughs to try on a harder difficulty.
In MMOs, there's a social stigma attached to "easy mode" content, and the games generally do a poor job of matching players to appropriately satisfying difficulty. With the implementation of "Dungeon Finders" and "Raid Finders" in modern MMOs, such as World of Warcraft, it actually reaches disaster point: the finder only supports a single difficulty, one that's often too hard, or too easy, for the majority of people jumping into the queue.
Further, even though MMOs claim to have multiple difficulties, they really don't, because your gear is tied to the level at which you kill content. Take a look at my WoW character for instance:
There's a huge difference between my gear, and the gear of a player who has only done "easy" (LFR) difficulty. Since there is a gear discrepancy built into the game, that means as additional content is released, my difficulty is further muddled - what would be hard content for an LFR player (based on gear alone) will be easy content for a player who has done previous Heroic raids and has access to more gear.
I'm opposed to the implementation of a wow-like LFD or LFR tool for precisely this reason. The core problem isn't the finder tool or the fact that it makes it easier to group (though there are social issues involved) the problem is that finders are "one side fits all", and when your audience is one million or more players, it's impossible that the content can really fit every player.
Even if it could, MMOs intentionally build in skill and gear gaps so that you can feel like your character gets more powerful over time, and these gaps greatly impact how difficult the content will be to complete.
I don't know of a perfectly elegant solution, but until a way can be found to appropriately match players to difficulty, and more importantly, to actually let players do the content of the difficulty they would like without feeling like they're "bad" or "gimping themselves" (huh... I only got green gear for this difficulty?) such a system can only lead to problems, as it will throw the "weekend warrior" into the same group with the "noob" and the "elitist jerk".
I'd be curious to see your insights into how an MMO can appropriately segment its population, the current method of "anyone who wants it hard should raid 40+ hours a week the first minute a piece of content drops, and everyone else can just be bored" certainly doesn't work for WoW... and I don't see it being the model SWTOR would base itself on in the future.