The dramatic nature of the book lies in its writing process: blinking the words, letter by letter. This must have been a tremendous effort, for both Bauby and the woman who did the actual writing. Apart from that, I’ve definitely read better prose and more interesting memoirs.
I suspect the film to be more dramatic and appealing (did anyone mention the book before the film’s success?) due to the force images provide; seeing the man captive in his locked-in-syndrome, desperately trying to communicate with the outside world, is clearly more emotional and sensational than just reading his thoughts.