In The Princess Promenade, Spike cheerfully tells Wysteria goodbye after he thinks she doesn't want to be a princess and begins to fall asleep; Wysteria says if he goes to sleep he'll nap for another thousand years—meaning she wouldn't live nearly long enough to see him wake up.
In The Runaway Rainbow, the loss of rainbows may not seem that big a deal—until you remember Sweetberry said rainbowberries grow only after the first rainbow of the season. Rarity unknowingly put Ponyville's food supply in jeopardy!
It's worse than that: the world would have lost all color. On top of that, if the Newborn Cuties are taken into account and we assume the bizarre creation of Sweetie Belle (she fell from the sky where two rainbows crossed) goes for all ponies, no more rainbows is nothing less than an extinction-level event. And just think - every season this must be done again. If they screw it up once, that generation is the world's last.
This profile◊ of Kimono's was written before G3 Unicorns were made, but it doesn't dispel the theories that something happened to separate the three Pony races.
In The Princess Promenade, Rainbow Dash seems unconcerned about Wysteria being a princess, thinking that she still is Wysteria. This makes sense when you consider Rainbow's British accent, since Britain is a constitutional monarchy, where the Royal Family is essentially The Artifact, with very little power. Of course a British character wouldn't find the title of princess to be a big deal.
Why didn't the founders of Ponyville even try to keep the layout of Ponyville◊ regular? Because, as A Very Minty Christmas establishes, the world's so small that jogging the north-south streets enough to keep them on their longitudes is a lost cause.