Victor Hugo's book is much darker than the ballet. I remember it being super descriptive of each scene. It described every place in such detail that you felt you were there, and there were descriptions of how the poor lived, etc. I think the irony was that Quasimodo was so ugly and physically gross, so people were scared of him. Meanwhile, Frollo was so evil on the inside despite being in such a "pure" position. Frollo was the one to be scared of, not Quasimodo.
I imagine a ballet that is as dark as the book would probably turn a lot of people off. It is a great book, but it has been so long since I read it.
Roland Wiley's excellent A Century of Russian Ballet 1810-1910 has a lot about the first Russian production of Esmeralda, staged by Perot when he was briefly ballet master for Russia in 1849 (the original Perot production actually premiered in London, five years earlier). There's quite a bit of translated reviews of the time as well as the original libretto, and it was mentioned that Quasimodo's role was diminished due to audience sensibilities, Frollo's death was off stage due to "women's delicate sensibilities", and that being a ballet it was decided it had to have a happy ending (which amuses me for two reasons, one is that even by then there had been plenty of tragic ballets, and the second is I have to wonder if they got in as much trouble from fans of Hugo for marrying Phoebus and Esmeralda at the end as Disney did for their animated version ). Of course this production is based on the Petipa revival notation of the 1899 production (he had previously revived and revised it in 1888 I believe) which is why already there's a fair amount of interpolated music by Drigo and others.
It really doesn't have much to do with the novel at all, just take some of the incidents, and key characters, and work from there--an awful lot like other ballet adaptations of the time (Don Quixote being the obvious esample which contains even less of the novel's plot). I haven't read the novel since I was in French immersion school as a teenager (I loved it though, even more than the other Hugo we read, Les Miserables), but I'm pretty sure it ends with Esmeralda being sentenced to death, and the final scene is a bereaved Quasimodo, clinging to her dead corpse so long as it whittles away to bones...
I thought it was GREAT--this was my first Bolshoi broadcast (I sadly missed Coppelia last Spring and hope it will come to DVD) and I was thrilled. it's especially exciting to me to see, because as a kid of 7 or 8 I would always read a library copy of Cyril Beaumont's Complete Book of Ballet which contained these strange sounding librettos for dozens of ballets I thought I would never see performed, La Esmeralda being one of them). I don't have any specifics to add, though I think it was a brave choice to pick this one to air, as it probably wouldn't be as much of a crowd pleaser to a newbie expecting Swan Lake.