Clearly, Mirror's Edge isn't my favorite game this year. But that's not to say it was all bad. As Brinstar has detailed, the main character, Faith, embodies a number of positive traits. Sadly, I think she also highlights the real problem of designing "franchise" characters: one isn't enough.
Here are the good points: Faith is an Asian-American woman, but her race and gender aren't mentioned in or relevant to the story at all. She dresses sensibly, given that she's in a pretty athletic career, and her body shape also reflects that without being overtly sexualized. She's stubborn, but not snarky (there's nothing worse than a "clever" character written by not-so-clever writers). Perhaps, you might be tempted to say, she's still a little rough around the edges.
The problem is that there might be one decent character there, but there's really no-one else for her to interact with, so those rough edges are never really sharpened. Everyone else in the game is dishwater-dull, from the other messengers to the cops to Faith's sister. The same goes for the dystopian setting--it's vague to the point of nonexistence. That's bad for the players, but it's worse for the protagonist: without anything to push back against, Faith doesn't have much ground to define herself. She has no points on which to take a stand, except for the most basic (I think most of us are anti-framing and anti-betrayal).
There are two trends in game writing at the moment: strongly-defined protagonists, a la Jade from BG&E, or mute stand-ins like Samus Aran. In the latter case, the surrounding world has to be made as interesting as possible. It's not a coincidence that the Metroid Prime titles tag everything in sight with scannable text, or that Half-Life 2 devotes so much work to giving the Combine little bits of "business," like their constant radio chatter and introductory set pieces. On the other hand, if you're going to make the protagonist an actual character, you can probably get away with a less defined world (BG&E's setting does what it's supposed to do, and not much else), but you'd better have someone for that main character to talk to, and their actions had better be strongly tied to concrete, interesting motivations. Jade, for example, is constantly interacting with her companions, and she clearly has strong opinions about each of them. Faith has neither--both her world and her friends are generic--and as a result, she herself is uninteresting.
It's a shame, because as I played Mirror's Edge I was reminded strongly of William Gibson's Virtual Light. Like the game, Gibson's book concerns a city messenger and outsider who gets tangled up in a class struggle. But Chevette Washington (Gibson's protagonist) is surrounded by interesting people: Rydell the reluctant rentacop, Sammy Sal and Bunny the bike messengers, and the eccentrics living on the ruined Golden Gate bridge. Chevette is not only defined for the reader by her interactions with these characters, she kicks off the plot herself when she impulsively steals the titular virtual light glasses from a sleazeball partygoer--at every step of the way, we're learning something about her. Faith never even displays that much initiative: her story really begins when her sister is framed, and she spends the rest of the game reacting to events.
Storytelling in games is like the weather: everyone complains about it, but nobody does much of anything about it. Progress has been slow, but (as opposed to other media) it's often shaped by the technology and culture surrounding it. One of the advantages I see from widespread console multiplayer is that it may build support for ensemble casts, as opposed to mascot characters. Gears of War, for example, is nobody's idea of a well-rounded drama, but its characters are inarguably much more lively than Faith is. Mirror's Edge gets caught on the wrong side of this trend for a variety of reasons: the first-person perspective, emphasis on time trials, and a primary mechanic of player-vs.-environment. I'm not sure that better characters would have saved the game entirely--it's got plenty of its own issues, as I've noted--but they probably would have made its failure a lot less aggravating.