That is a hard question because with relation to games, "right" and "wrong" are subjective. WoW arguably did more things "right" because the proof is in the pudding. WoW continues to thrive while RO limps along.
What did WoW do right, then, and what did RO do wrong? I think the answer is in that WoW has never betrayed it's core tenets. What made WoW the game it was is still the same thing that makes WoW the game it is. However, what made RO the game it was, no exists.
On the other hand, people who quit WoW eventually get over it and move on, you can be finished with the game. Anyone who quit RO, you know this, you never get over it, and in your heart, you know that if certain conditions are met, you would go back and play it.
I explained this to someone and it was hard to express in words, I will try to keep it shorter in text, but there are many elements to it.
The core tenet of what RO did right was Keep it simple, stupid.
RO had(s) some of the most complex game mechanics to date. The gameplay itself is quite simple. There were no separate skill cooldowns to manage. Despite the graphics, the game played much like Diablo as a hack and slash dungeon crawler. You want levels? Kill things. You want lore, do a quest (the lore is your reward, and not much else). You want to socialize? Sit in town and talk.
Compare the UI of RO to most other mmos. How much info does the UI in WoW, GW2, Tera, or anything present you with? It is a flood! You can't pay attention to any of it because you are trying to pay attention to all of it, because it is all potentially relevant.
RO's UI presents, in it's simplest form:
- The terrain.
The characters on it (you, your allies, npc, enemies)
Your Hp/Mp bar
The Hp bars of your allies
The chat window
Your Stat miniwindow (hp/sp/wt/lvl+%/z)
Socialization was better in RO because it lacked global chat. If you wanted to talk to someone, there was global party, private, and guild chat. The lack of global LFG means you only communicate with people you know. Yes, this creates the disadvantage of more "cliques" but it also more closely resembles human social interaction. In LFG enabled mmos, it can be like everyone on the street is yelling at you. In RO, talking to someone usually meant actually sitting next to them and talking.
The simpler UI and game that RO was made you feel like you were your character. There was not a flood of information in the window between you and your character. In a way, the character you played was... you. Like, if your body had a part of itself that reached through the internet and existed online. Just the UI in many other games makes you realize that you are yourself, and the thing on the screen is your puppet which you are operating and playing. Maybe that is just the effect of the first mmo you play (though RO was not the first MMO I played, and I feel that way).
RO's simplicity led it to have many problems, but it also granted it the charm that cannot be forgotten.