As a bird in a cage witnessing the resolution of "Cinderella," the player is an observer with very limited control over unfolding events.
For me, the sidelining of the player and the corresponding independence of conversation created a potent illusion of a deep world. Beyond this, the game was made enjoyable by clear options, a good set of endings, the ability to mess with the characters, and wickedly droll humor. But these are just good craftsmanship, and icing on the cake; there really isn't much to Glass beyond a little experimental envelope-pushing with the player's role and choices. For players, this is maybe thirty minutes of entertainment, light as meringue.
For authors, it may be something more. Limiting the player's control of events is not new (cf. Rameses), but I believe that the illusion of depth produced here is a significant technical breakthrough for NPC interaction complementary to those explored in Short's Galatea (and faintly reminiscent of what was so successful about Bob in She's Got a Thing for Spring).
In any case, this little game is as gratifyingly virtuosic as it is trivial.
This short, stylized and evocative "in your apartment" game is carried by technical merit and an effective surprise turn in the plot(Spoiler - click to show) -- a bizarre and horrifying disintegration of reality which reminded me of Philip K. Dick's oeuvre (e.g. Ubik, Electric Ant).
Shade is, however, marred by a few superficial defects. Being constrained to the apartment and an inexorably linear plot contributes to the game's feeling of airless claustrophobia, making it easy to excuse its minimal setting and choices. Gameplay generally flows well and is polite to the player; I only got stuck a few times, briefly, and never irreparably. But when I did get stuck, advancing the plot was often tedious, requiring systematic sweeps of the apartment to find the next trigger. For me, this compromised the effectiveness of the work by slowing the pace and focusing my attention on the manipulation of the parser.
I also felt that Shade would have been more effective and satisfying if the surreal plot, and particularly the ending, had sustained explanation more clearly than it did. As it stood, the events seemed arbitrary most of the way through, and I came away feeling that a lot of technical ability and conceptual cleverness had been deployed for no very compelling narrative purpose.
For me, the game's principal virtue was to demonstrate innovative tricks in the medium. But I think it is likely that readers' tastes will differ. Fans of mind games and psychological horror will find the game worthwhile for its craftsmanship and verve - and, in any case, Shade is so short and widely admired that most readers will find it worthwhile.