Likely one of the biggest changes that has gotten cheers is the boost to drop rates in instanced dungeons, including a big increase to the final boss mobs. For players who have made repeated instance runs without any final reward, those days are over!
Aion? Online: The Official Aion Fantasy MMORPG Website - PowerWikiQuote:
We are bringing the Aion servers down at 3:30 a.m. PST, November 18, 2009
for scheduled maintenance and the implementation of the 1.5.1 game update.
The maintenance is estimated to take no longer than 4 hours.
(via tentonhammer, thx!)