Glass is a Composition Type which bounces almost all magic spells back on the caster. The only exception is Shatter, which has a 1 in 5 chance of being cast successfully. When successful, it is automatically fatal to the glass creature; when it fails, it is automatically fatal to the player (though if the player is made of limestone, the spell might be transformed into another--hopefully less deadly--one.)
Glass creatures are useful because they can also reflect Transmog back on the player, allowing them to exchange their fragile flesh for something more substantial (except Stone and Glass). A player could, for example, cast a Crystal Transmog on a glass creature and then cast Shatter; if the attempt failed, it would not be automatically fatal due to the very high magic failure rate against crystal creatures.