Thanks for reminding me about this rather brilliant work which I haven't heard in a long time.
Now, genre classification is always a tricky business and this piece is indeed hard to classify, as it is rather unique in its combination of multiple genres; there are elements of at least (that I can think of) disco-sound, progressive rock, and electronic music. Other than the music being simply quite good, for me that's the reason why this album is so good and so interesting.
I can't think of any other work with similar sound and combination of elements (and I certainly don't know about any other conceptual album, almost a rock opera, made in disco-sound :-).
I think there are some examples of disco-sound with a similar sound, specially music produced by Giorgio Moroder (of Donna Summer and "I Love to Love You Baby" fame). But parts here and there, not a full album, and most disco-sound music (like for example the also excellent music of Chic) has rather different feeling and sound, despite the common rhythmic and instrumental elements.
If you're willing to do without the disco-sound rhythmic element and focus more on the sound, then Jean Michel Jarre's "Oxygene" (from 1976, 2 years before WOTW) is probably a good bet.
Now, if what you're really looking for is the conceptual element and are willing to try different musical approaches and styles, then there's plenty to choose from, here are a few suggestions, all different in style:
Rick Wakeman's "Journey to the Center of the Earth", or indeed any of the first 4 Wakeman solo albums, but I would start with this one, which is the second. Despite the difference in style it has many of the elements of Wayne's WOTW: it is a musical interpretation of a (well known) literary work, it has a voice over narration, it has an orchestra it has electronic elements. The music is superb.
Pink Floyd's "The Wall" is the ultimate "kind of" rock opera.
Andrew Lloyd Weber's "Jesus Christ Superstar" is the ultimate actual rock opera.
Camel's "The Snow Goose" is entirely instrumental, progressive rock of the best kind, but it is structured like a programatic orchestral suite that tells a story with musical elements (say, like Prokofieff's Peter and the Wolf).