I think one of the main reasons for the great success of RO was the lack of real competition by the time it became world wide known, perhaps that is one of the reasons for its downfall too. Nowadays you have a new MMO almost every single month, and unless you have something really unique, your game will not live long enough to see a second/third year on the market. So the bar wasn't really set too high when Gravity developed it.
Well, the game itself wasn't half bad. A lot of jobs, equipments and cards, providing the player the opportunity to make an unique character. There was room for all sorts of players. You like to RP? Cool, the game has an interesting story and a lot of quests, enjoy. You're into PvP? Okay, we have a chaotic (and yet really fun) GvG system, besides the usual PvP rooms. You're more into PvE? Oh, what about this big ass world we provide, with tough mini-bosses and challenging bosses which you can hunt with your friends.
The bots may have been the big issue... in the beginning. Now I think they're the only thing holding the official servers together. The main reason IMO is the lack of, I don't know, game designers? The game came out just fine. It was fun, balanced, interesting. But then they started to introduce a lot of new stuff. It's good to have updates, this keeps the game alive. Some of the new stuff are good. New maps, new mobs, new quests, new systems (I loved the battleground when it was released, I lived there for months). But then you have new classes, new (unbalanced) equipments, cash overpowered items. Unbalanced change after unbalanced change, cash digging after cash digging, the once game of strategic and competitive gameplay became a one-hit-KO game made for those who have money to spend on it. Plain and boring...