Graphics Tech People - why do mouth animations look corny, and what fix?

[Crazymann]

I was looking at the fine looking video of Heavy Rain, and (whilst it looks good), the mouth animations are corny, and the teeth are borderline creepy.  This is a really common issue.  ALL of the super heavily hyped games suffer from this - even the recent ones like Killzone 2, This got me thinking....

From a techical standpoint, what are the hurdles to getting the mouth animations to actually look realistic - with non creepy looking teeth especially?

How soon can we expect to see this?

Any ideas?

Thanks!