These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: The Rime of the Ancient Mariner
Fridge Brilliance: The Mariner speaks in such an archaic dialect because he never kept up with English. Keep in mind he's immortal, and wanders the earth.
Except the Wedding Guest uses just as many 'thee's and 'thou's at the few points where he says anything.
Let's not forget that this poem was written way back in the 1700s, so that may just have been normal speech of the day.