There's a lot of new content coming in the update, none of it bigger than the new Academy Bootcamp and Araka instances (both of which sport a level 50 entrance requirement). The former is a bit of a departure from the game's existing instances given that it's set in a circular arena and features "wave after wave" of monsters. The Araka farm instance requires an eye for quest dialogue detail, so pay attention to the Reian tribesmen associated with the dungeon.
The 2.5 patch is also adding a new divinity endowment enhancement system as well as several tweaks to the game's crafting and gathering systems. The endowment system adds various bonuses to your equipment that are then consumed in battle, while more than 300 advanced designs have been added with the aim of reducing the pain inherent in the higher-level crafting game.
Finally, 2.5 is adding new pets, an extensive pet mood system, group guide mentoring functionality, new temporary titles, new coin quests, new graphics engine options, new social clothing, and various legion enhancements. Check the patch notes on the official Aion website for the ful list of additions and tweaks.