Our speaker sounds as if she's (and we're just assuming it's a she) keeping things relatively casual, informal, and without all the frills of super-ornate language. Her ambiguity in words like "certain," "imperial affliction," and "internal difference" serves to reinforce the poem's mystery and intrigue without boxing that elusive "Slant of light" into any specific definitions. And her use of contractions like "there's" also tells us that her diction isn't going to be very formal or flowery. It's as if she prefers to depict that light for us in a way that's more natural, without any sort of forced effort.
She also speaks in a first-person point of view while inviting us to imagine this "light" with her by using words like "we" and "us." So there's also something that's kind of omniscient about her voice, even though she's speaking from a more personal perspective. And yet, she doesn't give us too much information about how exactly she's feeling, aside from some rather ambiguous descriptions of a "Heavenly Hurt" and "seal Despair."
But despite the speaker's lack of specifics, we still feel her pain. Even if we can't see or feel the light in exactly the same way the speaker does, we do sense the kind of affliction and oppression that comes with it. Her halting rhythms, coupled with the use of those dashes, also serves to create a sort of dissonance in her voice, as if she too is struggling to come to terms with those "Meanings" and her "internal difference." But we also notice that it's not as if the speaker is crying over her words. She may be feeling some despair, but we don't get any interjections like "Oh!" or "Lo!" Instead her pain is more "internal" and therefore a bit more understated which fits well the poem's mystery and intrigue.