I think another important thing to consider is how the makers of the film will try to balance the varying aspects of the film. After all, it's not only a musical drama, but also a thriller, a mystery, a 'quest' story, and more.
I remember watching a 'behind-the-scenes' feature for the new Battlestar Galactica series where one of the creators said it's important to remember that Sci-Fi isn't a genre in and of itself; it's a setting. So if you're making a Sci-Fi movie, all that means is that it takes place in space or features technology that hasn't been invented yet. You still have to decide whether it's going to be an action film, a romance, a psychological thriller, or whatever. I think the same is true of musicals. How many songs a film has doesn't really matter; if the writing, acting, and directing are good and able to work together well, word will spread and people will come to see it.
And of course, with a film adaptation of a stage show that has quite a few die-hard fans, they have to find a balance -- they want a film that will please the fans without being inaccessible to those who've never heard of the show. Catchy songs are one way to reach new audiences, casting popular actors is another... but the most successful way is simply by having a good story to tell, which I think J&H does as long as they don't mess around too much with the script.
The show was quite successful on Broadway even though many of us consider that version to be inferior, so I think the film will have some staying power as long as they avoid the mistakes made by the recent musical films that weren't so good. It can't be basically a filmed stage performance (like RENT and The Producers), nor can it ignore the heart and darker aspects of the story (like Phantom of the Opera). Chicago, Hairspray, and Sweeney Todd managed to find that balance, so I think whoever Frank and Steve get to direct this film should understand why those films worked, and find the right approach to help J&H succeed in its own way.