She is the feast, and I am the famine. Two of a kind, remarkably similar we are. When you look at her, you might see me, but that's your fault, I'm not there. one little straight line, you glazed right over.
If you replaced her with me, all would be gone, and many would be very unhappy.
Persons from "across the pond" (as THEY say) would have an easier time finding the pair of words.
Draught in British is equivalent to Draft in American, which has a meaning of: a serving of drink (usually alcoholic) drawn from a keg. In most feasts, alcoholic drinks are commonly served. Drought, basically shortage of water which can cause Famine.
The difference would be the line in the letter $a$, which would be removed to form the letter $o$.
There are cooking techniques which uses beer/alcoholic drinks as glaze over ham, etc.
Change draught (drinks/beverages) to drought (shortage) would definitely cause everyone unhappiness.
dessert (she) vs desert (you)?
dessert is a type of food, deserts produce very little food.
it is a common spelling mistake to confuse the two.
the extra 's' is a single line.
common to certain types of desserts.
there won't be much left on earth if each dessert is replaced with a desert of any reasonable size. The sweet-toothed would be very unhappy, though exchanging almost any climate for a desert would be unpleasant for many.
F(L)OOD. The extra line is the letter l, leading to famine. Being American or British doesn't make much difference, unless you look down at all the water, as you cross the pind.
it seemed to me that T and L were source of this confusion.
I find that 'cross the pond is used in DC and in London.
Yet rhyme and look alike to the (too) hasty eye or ear.
But the answer to the last line seems to make it a sure bet.
To those who gathered to partake in this replete repast.
So that's my guess, no more, no less, and this line is my last.
feast/least. "Two of a kind, remarkably similar": the words both look and sound alike. When you skim-read ("glazed over") the word 'feast', you might miss the cross-stroke ("one little straight line") on the f and instead read an l, resulting in least. And if you replace a feast with the least - the least being nothing at all - then "all would be gone, and many would be very unhappy".
Hint 1: the sound of the word should be more distinguishable in the UK/US than in the US/UK. I learnt British English and feast/least rhyme for me; I don't know how they're pronounced in the US, but have learned in the past that pronounciations can vary wildly. Hint 2: the little line that's missing isn't a letter of the alphabet on its own, but is strongly hinted to be part of the letter; in this case the cross-stroke on the f.
She is fat and you are flat. One little line - the letter l - is the difference between you.
 So apparently it's not a letter that's missing. Which makes me think that it's a T going to an L.
Okay, none of these is really a spoiler, because that would imply that they might be right. I guess I could have missed the word pair in my quick look through the pastebin, but perhaps it's not a T->L. It could be T->I, or even I->T (depending on your font). Or M -> N or A->H.
Which US city is it?
Will this be my final Riley Riddle?