Based on the table shown in the notes, quest experience is being increased; to the tune of almost 500% on average (if our calculations are correct). The craziest example is a level 44 quest going from 13,300 to 703,820 experience (53 times as much).
Extracting aether now has a chance to debuff you with "Aether Venom." This debuff reduces your chance to successfully extract a gatherable down to 0. Players can remove this effect with a "Word of Purification" ability. Each character has this ability but to successfully use it, one must properly type in a captcha. This seems like an obvious attempt to prevent bots from gathering up all the game's resources.
Finally, Spiritmaster pets can now fly. We're not quite sure how this issue ever made it into the game at launch (design oversight or complication?), but 1.5.1 aims to finally remedy it so Spiritmasters aren't totally gimped in aerial PvP. Be sure to check out the full notes here.
[Update: These notes are for the Korean test server, so they're not final notes for the Western version.]