Planetside makes no pretense of being an RPG, for instance, while Second Life is really a virtual environment rather than a game per se. But the roots of the term do define what most MMOs are fundamentally aiming at: being an RPG with a massive component of players. Which is ironic, because as We Fly Spitfires points out, they're not really RPGs at all. They feature the stats and the leveling, but nothing of the larger sense of place and story that the genre hails as its strengths.
Compared to games such as Dragon Age: Origins or Oblivion, it's clear that for all the strength of options we might have in some MMOs, we lack any sense of real character specialization or unique progression. Part of the concern, of course, is content -- you don't want to necessarily force any player who didn't happen to make the right choices in a dialogue tree to miss out on a major endgame event. The ubiquity of communication also helps herd players toward a specific set of specializations or ability tree, with little to no deviation encouraged.
But there's more that can be done, and games such as Star Wars: The Old Republic seem to have a greater intent to focus on individual story and progression. Even if you don't necessarily like the impromptu acting which is usually associated with roleplaying in the genre, it's hard to deny that a greater sense of individual choice and uniqueness would be intoxicating.