-It remains one kingdom in the eyes of the Iron Throne as Lord Paramount Denethor II Hurin of Gondor descends from a line of stewards and the Kings of Arnor have abdicated in favor of living as clansmen (House Isildurion lives, worry not), leaving Lord Elrond Peredhel to watch over the land in their stead. The Starks of Rhovanion bent the knee, and so all three were preserved as one kingdom but three Lords Paramount.
-Elves, dwarves, hobbits, and ents still dwell here, but elves have been greatly weakened by the transposition of Middle Earth; many fear they will be unable to travel to Valinor. The Shire is the breadbasket of the North, though they have had to adapt to harsher winters than before. The dwarves retreated deeper into their mountains--they do not trust a kingdom built on dragonfire, but still trade with humans.
-Some Westerosi houses found themselves ruling over new castles: House Dustin proudly rules from Isenguard, House Tallhart governs the ruins of Ost-in-Edhil, House Ryswell holds Tharbad. Even a few Riverlands houses got snagged--House Mallister proudly holds Seaguard-by-Linhir, and to everyone's great dismay, the Twins of House Frey stand in Tumladen, controlling the crossings of the Sirith.
-Many rivers have been redirected: the Celduin and Upper Anduin have merged into the White Knife, while the Carnen coincidentally follows the path of the Weeping Water starting from the Dreadfort. The Upper Green Fork has merged with the Nindalf and Mouths of the Entwash to recreate the wetlands of the Neck, depriving the Trident much of its former water outflow.
-Thanks to the crossover, the Vale finds itself swarmed by orcs and trolls as it becomes the equivalent of Mordor. Similarly, the Corsairs of Umbar have found a snug haven across from their fellow pirate brethren the ironborn in what was once the Banefort and the Crag. Not all the news is bad however--the Dorwinrim have adapted to their strange new climate and adapted their wines as well, making House Hornwood wealthy enough to resist falling under Bolton vassalage.