Franchise Killer: It would've been this if the game sold fewer than a quarter million units. (More in a retroactive sense; the game was expected to be the final game in the series and was developed as such.) Thankfully, this was was averted, as it became one of the best selling titles in the franchise, and was a commercial and critical success in Japan and the West.
I Knew It: When Male Morgan was confirmed as the boss of the first Future Past DLC Chapter, some fans theorized that Female Morgan would be the boss of the second chapter. They were right. Even less people were surprised that the third and final Future Past DLC Chapter's boss turned out to be Grima.
Jossed: It has been theorised that Lon'qu, the swordsman who Cannot Talk to Women, would extend his awkwardness around them to his own prospect daughters from the future, should he marry a first generation character who gives birth to a girl. The Future Past DLC josses this: if he marries Tharja and then he goes rescue the alternate universe!Noire, AU!Noire bluntly mentions his aversion to women, and Lon'qu's reply is basically "why would I be scared of my own daughter?". Granted, conversations with other potential daughters suggest that while he is unafraid of being near his daughter, he was afraid that he might.
Killer App: It was reported in the UK that the bundle of Awakening and a limited edition 3DS XL made up about 30% of all 3DS XL sales the week it was released, and helped boost sales of the system 50% compared to the previous weeks.
Marth Debuted in Smash Bros.: Thanks to the SpotPass optional battles and paid DLC, HEAPS of of characters from FE2, 3/12, 4, 5, and 6 are appearing in English for the first time ever in this game. Marth himself averts this, having previously appeared in Shadow Dragon.
Newbie Boom: The game got more marketing in the West than previous titles, giving it much better mainstream exposure than its predecessors. It went on to become the best selling Fire Emblem of all time, which in turn drew in many new players to the fandom. Robin and Lucina's inclusion as playable characters in Super Smash Bros for 3DS and Wii U a few years later sparked another small surge of new fans due to the Smash's series popularity.
No Dub for You: While the various European localizations are very creative and heavily Woolseyized, only English and Japanese voiceovers exist for the game, unlike Radiant Dawn's multiple European voices. However, all Western versions of the game contain both voice tracks, unlike with previous Fire Emblem games.
One of Us: If his Twitter is anything to go by, Kaiji Tang, the English voice actor for Owain, is an avid player of the game himself.
Subverted with Lucina and Lon'qu, though. Even though their voice actors (Laura Bailey and Travis Willingham, respectively) are married in real life, they can't be paired up themselves because the only character who can marry across generations is the Avatar.
Serendipity Writes the Plot: Why Morgan has amnesia, as, depending on whether the Avatar marries a first or second generation unit, Morgan would have wildly different backgrounds and would require numerous script changes to their Supports. Amnesia easily circumvents this problem.
Talking to Himself: Insofar as Voice Grunting allows, given doubling up among both casts. Especially notable though with the mother-daughter pairs Sumia and Cynthia and Cordelia and Severa in the respective dubs (Ayano Yamamoto and Julie Ann Taylor respectively). Also Donnel and Stahl share the same English actor.
Some of the fathers and sons can end up talking to themselves if you pair the mothers up appropriately: Lon'qu and Brady, Kellam and Gerome, Gregor and Laurent, or Yarne and Robin (if the right voice option is picked on Robin).
Several unused sprites for characters exist showing that Sully, Stahl, Lon'qu, Olivia, and Say'ri were able to promote into the Hero class at some point. Since all of those characters can become Myrmidons, this implies that the Hero used to be the promoted form of the Myrmidon.
A critical/skill portrait was discovered for Phila was well; unfortunately, though, she's killed off before she has a chance to use it.
The game's files list the names Andrea, Leopold, Bella, and Tracie among the game's characters. Although their names are all that is left of them in the game, these were apparently characters of some kind that were Dummied Out.