Continuous added content and highly customizable stats and builds has kept me playing this MMORPG for 7+ years!
IRO now has a free server to play, called Valkyrie, and has implemented many repeatable quests to get to the higher levels much quicker to prevent long hours of endless grinding. Thanks to dedicated CMs, game events are much more appealing and numerous than they were several years ago.
Since 3rd jobs will be added soon, along with a complete overhaul of game mechanics to balance out the upcoming level cap of 150, Ragnarok Online still has a many years of updates and added content. I'll probably still be playing this online game for years.