High Five features a reduced XP requirement for characters at level 78 or above, and clan levels 6, 7, and 8 will see a similar adjustment in terms of reputation points and member number requirements. The update is also bringing a ton of new character skills to Aden, as well as an extensive list of changes to existing skills. New quests and hunting zone revamps are also in the works (specifically Antharas' Lair and Dragon Valley), and numerous weapons, armor, and jewelry can now be purchased at the Giran Luxury Shop with Adena instead of crystals.
Finally, NCsoft has made significant adjustments to the Grand Olympiad system, including revamping the arenas to instanced zones (allowing for 160 simultaneous stadium matches), as well as various tweaks to both participatory and spectator aspects of the system. You can check out the preliminary patch notes on the official forums, and don't forget to read up on the particulars of this week's test server implementation.
[Thanks to Justin for the tip!]