Valjean, Javert, Eponine, Fantine, Marius, The Thenadiers, Enjolras/Student Revolutionary, Gavroche, Montrieul Whore, Montrieul Factory Worker, Factory Foreman, A Poor Person, Army General. Cosette
She gets a lot of stage time - she doesn't die off like [spoiler] Fantine or Eponine - and she doesn't have to wear a costume that is, at best, slutty (Mdm Thenadier). She's adored by the men of the show - even Javert rides out to her at the Thenadiers, if only to nab her as bait for Valjean. And she gets to duet with Marius who tends to be the most attractive man on stage, aside from the red-blooded Enjolras, of course.
Yet she's still considered the duff role.
So come on. What's wrong with Cosette?
No, the difference between Cosette and Valjean is that while Valjean has strength - physical and emotional - Cosette's basically a drip.
It doesn't help that her vocal range is set so high either. Because Valjean is always redeemed for me by the gorgeousness of his songs and vocals. Despite being a saint, these give him density and dimension. It's tiresome that he's always worrying he's not really very good at all, but at least he's tiresome whilst singing songs with a bit of welly.
Cosette, on the other hand, sings somewhere in the realms of a dog whistle and offers very little insight into her inner life. In My Life is basically about how she doesn't know anything. The girl's a total void.
And why? Because she has been kept in the dark about practically everything in her life. She has been the catalyst for so much yet she's completely oblivious. Amanda Seyfried's interpretation of this is excellent (even her Disney-esque c.1937 vocals are perfect for that bird-like Victorian heroine) - she brings not the usual child-like innocence to the role but a sense of complete bewilderment - a girl who's a completely unknown quantity, even to herself.
The discovery of Cosette's existence when he saves Fantine stops Valjean from turning himself into Javert.
Point 2: She gives Valjean a reason for lifving
Cosette makes Valjean realise what he wants - she gives him purpose. Admitedly to be a dullard, but still ... it's better than going back to being 24601.
Point 3: She saves Marius's life
Because Marius falls in love with her, Valjean saves Marius from the barricades. If Marius had chosen Eponine, he'd be dead with all his friends.
Point 4: She saves Eponine from a life of woe
Okay, this is kind of tenuous, but it took Marius falling in love with Cosette for her to realise it was never going to happen. And, okay, she dies before discovering that actually, there's more to life than a dreamer and hitching herself to someone more worthy's wagon (ie a man who doesn't fall in love in the blink of an eye), but perhaps what awaited her was worse. I mean, the Thenadiers are her parents after all ...
Point 5: She gives Javert a reason to kill himself
Because Valjean feels compelled to rescue Marius for Cosette, Valjean bargains with Javert for more time to make sure Marius lives. Javert lets him go because of this. And then jumps into the river. If Valjean had emerged from the sewer alone, Javert would've had him for sure.
You see - she is the ULTIMATE plot point! It all turns on Cosette. That's pretty important.
So perhaps it isn't about Cosette herself that we're so nnnghhhh. Maybe it's something in the way she's presented to us?
In short: perhaps it's time someone allowed her to be played with some balls?