Cross-Faction

Turtle-WoW implements cross-faction features, allowing horde and alliance to join parties together, do dungeons and raids, trade, mail, and every Auction House in the game is also merged into one, so horde and alliance can easily trade for a single server economy.

NPC reputations are still in place, so alliance players are still permanently hated by horde NPCs and vice versa. This results in many quest rewards and reputation items specific to factions remaining exclusive for that faction. PVP is also not cross faction; battlegrounds remain horde vs alliance. As for world PVP in xfaction parties, PVP within them is fine, however faction A healing a faction B player is prohibited by the server rules.

Gameplay Implications
This is really the first time we get to see, on a vanilla server, shamans and paladins on the same team for dungeons and raiding. This allows for powerful combinations of buffs, most notably both Blessing of Salvation + Windfury Totem for the meele groups.

However... unlike TBC, the following paladin and shaman abilities do not stack:

As you can see, totems will always overwrite blessings, even if the blessing effect is better, which leads raids to forgo tranquil air, mana spring and mana tide in most circumstances.

This means that for the most part, raids will choose to play by either the 1.12 alliance meta with only a few resto shamans and all pally buffs, or the 1.12 horde meta with shamans in each group and only a few paladins for blessing of salvation, wisdom, and light and judges of wisdom and light. Shaman build diversity comes out significantly nerfed by these restrictions, most notably resto shamans should consider skipping their 31 point talent in raids and even in higher level dungeons due to its conflict with blessing of wisdom. Paladins also begin to contribute less damage to the raid the more shamans are brought since Windfury and Strength totem are favored more than Kings and Might. The resulting effect is that the two classes provide diminishing returns on each other the more the counterpart is brought, so effectively there remain 8 classes present in a raid, the 8th split between Shaman and Paladin. These issues are currently under consideration be resolved as part of an overall change to raid buffs, including world buffs,