For players, this means that there will be a lot more inter-faction communication and interaction. To start with, both factions will be able to join together for public groups, trading, the dungeon finder, raids, instant adventure, and weddings. The team also has plans to open up chat channels, quest sharing, guilds, friends list, and more between the two sides. There will be exceptions to some of these features on PvP servers, however.
RIFT is also preparing to implement PvP normalization, which means that all players competing in PvP will do so with the same stats and no benefits from gear. Trion Worlds promises that there will still be "meaningful PvP progression" but that this should put the emphasis on skill rather than gear.
[Thanks to Warbs and Marc for the tip!]