The game is light-hearted, cute and obviously anime-influenced. The only race available is human, but there are 6 Jobs that you can choose, each of which can later Job Change. At the moment, different clothing styles are not implemented, but you can pick a hair color and style, and your headgear affects your appearance. Like most asian MMORPGs, there's a whole lot of for-show headgear, i.e. stuff that looks cool and does little or nothing in terms of game effects.
There was a review of RO on Something Awful, which might tell you something about the game, although it's a somewhat inaccurate portrayal. Basically, RO sucks when you're a newbie. I mentioned 6 Jobs earlier... When you start out, you take the job "Novice". As a Novice, the only skills available for you to raise are "Novice Skills", which let you do basic things like start private chatrooms, sit to regenerate HP/MP twice as fast, and join or start parties. Most games would let you just start out with those abilities, but the makers of RO seem to think you should have to work for them. You have to gain all the Novice skills before you can change Job to one of the 6 main jobs. Depending on the stats you picked at the beginning, getting enough levels for those skills might take a while...
There are actually two kinds of levels and experience - Base and Job. When you gain a Base level, you gain points to raise your stats, and when you gain a Job level, you get a skill point. At Job level 40, you can change from a main job to the second job for that class, e.g. from Acolyte to Priest. Most monsters give more Base exp than Job, so you can expect a long hard road to the second job.
There's not all that much to do in the game other than kill monsters for money or exp. A LOT of people think that actual roleplaying of characters is geeky; it's one of the most non-RP MMORPGs I've seen.
The system also lends itself to various abuses. A pretty major one is that dying doesn't carry an xp penalty (at least in some localizations of the game), so it becomes a convenient way to go back to your spawn point. It's still pretty annoying if you die when you're really far away your spawn point.
The major abuse that gives people a lot of grief is the experience system. If you're not in a party, the experience gained from a monster is proportional to the fraction of its hitpoints you damaged it by. e.g., if 2 people damage a monster for half its hitpoints each, they'll each get 50% of the monster's XP value. If you're in a party, the experience is shared evenly by all members, but only if they are all within 5 base levels of each other. The non-party experience sharing wouldn't be that much of a problem, except for the monster-targeting AI. Most lower-level monsters are non-aggressive, and so are some of the of the higher-level ones -- they will only attack if you attack them first. This makes it easy for higher-level characters to hit a monster once, then take all the damage while a lower-level character attacks it. It also makes it easy for a player to wait until someone else hits a non-aggressive monster, then start attacking it and get a share of the xp at no risk. Aggressive monsters will often change targets when multiple people are hitting them, but people will still try to steal a share of the xp from them.
The other major abuse is "looting". Monsters drop items on the ground when you kill them; for a short time, the loot can only be picked up by the person who did the most damage to the monster. After that, it's free for the taking. The problem is that during busy periods, there is often a lag in the updating of game information from the server, so someone could easily be unable to pick up their items before the time period has expired. In that time, someone with less lag (or who's just lucky) could make off with a potentially very valuable item.