I agree with everything that's been said, but will add a couple more I've personally seen:
Cutting on the bias for half square triangles (argument over whether it will stretch or distort if you don't)
Whether fat quarters, dimes, nickels, or fat 8ths are worth purchasing.
I posted a pic of some thread painting I had done on my LA machine (this was on Facebook), and got a nasty back about "not everyone being able to afford a machine that costs as much as a car" (which wasn't even close to the truth; the price I paid for my Avante would be only 1/2 of the cheapest car out there, and more like only 1/3 the price of most "ordinary" cars these days.) The comment hurt anyway. She didn't even look at my art work. Or, maybe she did and was secretly jealous of that too.
Some people just like stirring up trouble, and some, like the 2nd example, just can't hide their jealousy. Doesn't matter what you can do with what you have, if you own something "nice" they will be venomous toward you for it. I think some people are just MEAN and think they feel better about themselves if they've put others down, and that can extend to any element of your quilt- color, choice of design, proportion, overall size, choice of borders, quilting style, yada, yada, yada. If they can nit-pick about it, it turns controversial as other try to undo what the mean person said. Not necessarily "quilt police" as much as "mean girls".
The same mean girl said you shouldn't be making quilts for premature babies, because they can't sleep with them anyway. (She quoted some pediatric recommendation for no soft items in the crib. Funny, she completely ignored the picture of how to tuck the blanket over the baby's lower half, in conditions where a quilt or blanket is necessary.)