A storyline and worthwhile quests.
No need to elaborate much on it. Just think of your first days, the ambiance was great, the music was enticing - hell, even job changing was fun and got you into the role of your job. I remember being a goddamn saint to everyone the first time I switched to knight. What kills this? Lack of a storyline. Ok, so I reached 2nd job, I go into this guild competition thing. What do I learn? RO is about grinding, about gear, about power. Ok, I can take that, but what am I exactly fighting for? Is there a point to massively obliterate monsters? Any plot to conquering castles? Any reason I should be helping this NPC? None. You just kill - because. In the end, the game gets to be the massive dramafest we know today. Where there is no entertainment to be had, people will fight each other to create it (been there, done that).
Then, quests. It's shockingly impressive that the quest system is based on "bring x amount of items", gravity sometimes doesn't even bother to add why are such items needed to forge a said item, and why in such a heavy amount. Also most quests are completely useless and a waste of time. While some bear some nice items, you can count the ones you remember how to do off of the top of your head. The rest? Oh you get apple: 1ea. Again, this brings me back to the storyline since worthwhile quests have some nice stories to it, but they never collide with one another. While the RO ambiance is set to be a large kingdom of various cities working with each other, you barely even see two towns connected with more than just an NPC you have to find to continue the quest.