Review by Andrew Clements here

"Kepler himself, sung by Martin Achrainer, is the only named character, but the opera's two acts contain nothing biographical, narrative, or even dramatic."

I agree that Kepler isn't that far away from an Oratorio.  Though I saw the piece in Brooklyn presented as a concert version and it worked, but seeing the stagted version fron Linz is better.  It's that same old argument about whether pieces like Einstein are actually operas. Glass' barometer of "do you have to do it in an opera house?  yes? ok, then it's an opera" is about as good a measure as anything else. 

Actually, it's a big part of Glass' way of writing operas – he presents a subject and a musical depiction of that subject, and it gives directors and future directors a lot of room to imagine their own version.  I wonder if Parsifal would get only two of five stars for its own lack of action and drama?