Ragnarok had a pretty simple system to follow from its original game. There was a whole lot of going into dungeons and grinding on monsters. Of course, there was also that "massively multiplayer" thing going on, which the DS can't really emulate. But for the most part, the feel of the game is identical, save for an added storyline.
Yes, this game has you playing through the story of a young novice adventurer. His motivations are simple: become a great adventurer and make his deceased mother proud. He also wants to start a party, level up, and start a guild to serve the same goals. If you get where this is going, basically your main character wants to get to the endgame of Ragnarok Online to make his mother proud. Cute, but also very convenient.
The writing in the game is very simple, and accessible. That's not to say it's not enjoyable, it's just that there's no flowery language, no big speeches. You might say it follows the spirit of the online game in this sense. The characters in this game are more apt to talk about how something "sucks" rather than brood for paragraphs.
Though you can't customize your main character's appearance, you can choose any job class and higher job path you like. You start out with the classic Novice class. From there, you can choose from the base classes of Swordsman, Magician, Archer, Thief, Acolyte, Merchant and Taekwon Kid. Higher level classes like Sage, Rogue, and the Bard/Dancer class aren't available, but the DS does get the exclusive Dark Knight and Shaman classes. Stat distribution is just as mysterious as the online game, but most classes are pretty easy to figure out. Your Mage needs intelligence, your Swordsman benefits from strength. Still, an online guide may be in order for creating the best character in higher level classes since the manual and the game fail to fully describe what each statistic means to each class. Then again, you level and gain money a lot faster than in the online counterpart, so it may not be a month between levels, and it's not such an important, character-crippling decision when you spend points.
The game consists of a bit of story, broken up by quests. The quests don't depart far from their online inspiration. You need to go collect a bunch of jellopy, then you need to bring them to a guy. The guy is probably going to tell you to go to another guy, who lives on the other side of a dangerous map full of monsters. It's generally designed to force you to level, so you can move on to the next collecting quest.
For the majority of the game, you'll command a party that includes your main character and 2 others. Later in the game, you'll be able to recruit different characters in your guild, but you'll always have that 3 character limit. There is a multiplayer option where you can play with up to 2 more people, as long as everyone has a copy of the game. Only here can you change your appearance, which helps break up the monotony of 3 red-haired guys running around. Don't worry much about missing out on this part of the game, as there is only one area in the game where you can play multiplayer.