Like wonderfulwizard11 said, demanding can b defined differently. Some role demands require differeant things. Everyone already knows and has said Elphaba is pretty beastly on the voice, but does get a good chunk off offstage time. No the character writing isn't 100% stellar, but it's a lot better than most other musicals that are based off a super popular source. I can only imagine the same will be said for the future role that will be 'Elsa'.
Elle...I won't lie and say it isnt 'fluff' acting, but lets remember it's a comedy, and an O' Keefe one at that. That's the kind of show he writes. But that role is pretty hard to sing actually. There's a reason Laura Belle got nodes from it. I hate Legally Blonde anyway, problem is it never should have been a musical, hence the 'fluff acting' stigma. Most shows that adapted from sources that should never be on stage to begin with tend to be that way.
Evita because duh. BEASTLY sing, but still must be acted well. J and H, obviously a beast for a man, still not well written but merely for the constant change in 'mindset' of the actor is toll taking for sure. Lucy is a hard sing honestly, not necessarily because it's belty but because it lies in pretty uncomfortable places in the female voice. Somewhat like Elphaba, but Lucy has even MORE offstage sime than Elphaba.
I just saw SoR and yeah, it's prrreetttyyyyy close to male Elphaba/Evita status. Dewey is barely ever offstage, and while he doesn't sing all over the place, the amount of energy needed for when he DOES sing it's crazy.
I've found roles that are considered demanding are written two different ways. The one that's almost never leaves the stage, but the energy is sustained throughout, it's not necessarily always on such a super high energy level (Monty, future Anna, Glinda to some respect, Mama Rose sort of, ValJean, it's just his vocals that are nuts, etc. or the role that while not always on stage, when they ARE it's 'go go go super high energy' almost the entire time, roles I like to call high-wire acts (Phantom, Elphaba, future Elsa, D'ysquith Family, Dewey is half of each kind in a weird way, Javert, etc.)
'Deamanding' roles seem to have certain qualities about them that MAKE them demanding whether it'd be vocally, emotionally, psychically, etc.