Good question, but it's worth pointing out that glitters in this context is a verb, not a noun. The subject of the sentence is all.
When all is used as the subject of a sentence, it functions as a pronoun, and it can function as a singular or a plural pronoun. From NOAD:
all (pronoun) [with clause ] the only thing (used for emphasis) : all I want is to be left alone.
(used to refer to surroundings or a situation in general) everything : all was well | it was all very strange.
So:
All that glitters is not gold
is roughly equivalent to
Everything that glitters is not gold
and the verb tense agrees.
but:
All who eat are not gluttons
is roughly equivalent to:
All the people who eat are not gluttons
so there is no problem there, either.