This article contains spoilers. But go ahead and read it anyways; it's not like you're going to be able to dodge this one.
The Twilight Alicorn Controversy often refers to one of the most chaotic fandom reactions to show canon. What started as a bunch of whacked out rumors that people believed were just trying to start drama. Well, that was until one day Hasbro decided to spill the news. Behold.
Origin of the Theory
Back in September, a few images floated around the MLP communities involving bunch of winged twilight plushies. That's where the rumors all started speculating. A few months later, A reddit user posed as a Hasbro employee to try and troll the community. While no one believed his story, it coincidentally became a reality.
The brink of the shitstorm took place when Entertainment Weekly promoted the Season 3 finale. First two boards that were quick to react were Derpibooru and 4chan's /mlp /. Then Ponychan's /pony /, EqD, and DHN caught wind of it. The response was either cautious optimism or widespread havoc.
The news was the fastest spreading story in the fandom, ever. Mods on Ponychan, had to work at their hardest, to keep everyone from panicking. Even the mods at 4chan had to take action to keep the board relatively organized.
A large aspect of the community, disdains the idea altogether. One of the biggest reasons, aside from the Mary-Sueish description of the story, was the direction of the girly "I wanna be a princess" stereotype.These are the same exact reasons which Lauren Faust intended to oppose when she was initially appointed as Executive Producer of the show.
While it still remains unconfirmed, it is also speculated that these writers were pressured by Hasbro in order to promote the toy line. Unfortunately, this also goes against what their representative stated at EQLA, they were aiming to make toys based on the show, not vice versa.
In regards to show canon, Twilight already has a sufficient ability to do magic anyhow. And in question of her promotion to "princess," she is not related to royalty anyhow.
Others were concerned whether she would be over-idealized and would hurt her character as part of the show.
"I Believe in M.A. Larson" Badges
Two days prior to the announcement, dA user, PixelKitties posted a purple-indigo badge that read "I believe in M.A. Larson", stylized with Twilight's cutie mark. This coincidental timing is what made it so popular, since M.A. Larson is the writer of the season finale episode.