Dummied Out: The Bionis' Shoulder, it was featured multiple times in early previews of the game, and the location is actually in the game data as a mostly fleshed out map. There were many theories on what purpose the map would have served, but was later confirmed to be meant for testing purposes and wasn't meant to be in the game. Averted as of the Definitive Edition, where the Bionis' Shoulder is part of the new epilogue, Xenoblade Chronicles: Future Connected.
The seventh party member is often referred to as "7th" or "Seven" to avoid spoiling that Fiora is revived as a Face Mechon and eventually rejoins the party. It also doubles as a Shout-Out to another blonde female cyborg, Seven of Nine from Star Trek: Voyager.
"Naked Dunban" for the common strategy of combining the effects of Dunban's Spiritual Awakening and Inner Peace skills with Melia's Unadorned Beauty to make him a very potent dodge tank when armorless.
"Duncan" began circulating for Dunban after a notorious review by IGN of Definitive Edition which initially pronounced his name that way (as well as pronouncing Reyn as "Rain").
Fanon: Some fans give Dunban and Fiora the surname "Uzuki," even though their actual surnames are never given (assuming they even have them, since most Homs have Only One Name). This is mostly a homage to Jin, Shion, and Citan Uzuki.
Jossed: Theories abound as to what purpose the Dummied Out Bionis Shoulder map might've originally served in the plot, with the most common being that the party would've crossed it to get from Makna Forest to Eryth Sea, that it was the home of the extinct Giants race, or the original location of Colony 6, and that it was cut for time/budget reasons. However interviews after the release of Definitive Edition revealed that the Bionis Shoulder was not cut as most people assumed, but rather that it was was an early map used for testing purposes, and that there were no plans for it to appear in any major fashion.
There was a theory that the Mechon that saved Juju and Otharon from the Telethia when Shulk and the party is going up against Dickson was actually Xord, due to sharing the same voice actor, though some were unsure that this was the case due to Xord exploding. The Monado Archives confirms that it was actually Xord◊.
As people completed Xenoblade Chronicles 2, there was an immediate suspect for where the dimentionally-lost Ontos core ended up. Definitive Edition confirmed this by changing Alvis' keyring necklace into what is clearly a dark red Aegis Core crystal.
Keep Circulating the Tapes: In the United States: The retail game is now out of print from its highly limited run by one retailer. This gives it a high value to collectors. This is likely the reason why it got a port on the New Nintendo 3DS. The Wii version has also since been released on the Wii U Virtual Console, in Europe on August 5, 2015, North America on April 28, 2016, and Japan on August 5, 2016. Used copies can be found at nearly any Gamestop, albeit still at close to original retail price. Eventually, the game was remastered/remade for the Nintendo Switch as the Definitive Edition.
Limited Special Collector's Ultimate Edition: Xenoblade Chronicles: Definite Edition for the Nintendo Switch had a collector's edition upon release. The ones in Japan and Europe had an artbook and a soundtrack selection (with Europe soundtrack being on vinyl), while North America got the artbook.
Newbie Boom: Newcomers started rushing in in late 2014/early 2015 partially because of the protagonist Shulk's inclusion in the fourth installment of the ever-popular Super Smash Bros. series, partially because of Chuggaaconroy playing it, and partially because it got a much-needed re-release on 3DS and the Wii U Virtual Console. Eventually, its own popularity and that of its sequel resulted in an Updated Re-release for the Switch.
Posthumous Credit: Although Gorō Naya (Odama), Takashi Taniguchi (Agares and Zanza) and Sayuri Yamauchi (Laulan) died years after the game's release on the Wii, they are still credited for the Definitive Edition release since it contains the original dialogue.
Sleeper Hit: The game didn't receive a lot of advertising or recognition by Nintendo upon release in Europe, but positive reviews and word-of-mouth (courtesy of Operation Rainfall's efforts) made the game sell rather well, to the point where they had shortages of available copies because of the surprisingly good sales. The fact that it's the Spiritual Successor to cult classicsXenogears and Xenosaga also helped.
There is are also a few explorable maps that were Dummied Out of the game (though still accessible by hacking). The most prominent of these are a large, elaborate map set on Bionis' left shoulder, and a map for the Colony 9 created after Zanza's defeat, implying that a post-game was to be included at some point in the game's development. Bionis' left shoulder is set to return in the Definitive Edition, finally averting this, though whether or not other cut content will make it is yet to be known.
Dickson, Mumkhar, and Alvis all have unused dialogue, including for attacking high level enemies (Mumkhar never rejoins the party after the Battle of Sword Valley, Dickson only joins in battle against Metal Face after Central Pit, and Alvis only fights when Shulk encounters the Telethia for the first time and gets Monado Purge), burst affinity quotes, retreat quotes, KO dialogue (Alvis can't die in his only fight in the game), and Shulk, Fiora and Reyn would have Tephra Cave battle lines exclusive to the Ether Cylinder quest.
Shulk originally had 6 swords before getting the Monado, including the Debug Sword which was, aesthetically, the True Monado, despite being treated like a regular sword, which was done to test out specific areas. Sharla would've also received Gadolt's Cannon after Mechonis Core.
There was originally going to be an enemy called 'boss', who had no animation when he was scrapped.