For anyone wanting skill based PvP, RO is your game.
RO is probably one of the best PvP and GvG MMO's out there. The sheer amounts of customization is astounding. Not only are you able to choose your stats (which, unlike a lot of MMO's with stat customization, have a HUGE impact on your character), skills which have to be chosen carefully (can't get every skill), and equipment.
Armor comes in Upper, middle, lower headgear, armour (body, basically), shield, garment, shoes. Each armor piece can have 1 slot. Middle slotted headgears are INCREDIBLY rare however, and lower headgear can not be slotted.
There are also two accessory slots. Accessories can be 1 slotted, or no slot but have its own effect.
Weapons can be up to 4 slots.
The more slots a weapon has max, generally the weaker or more common it is. Rare weapons can often not get a slot, or at max get 1 slot. (For a lot of money, and a very low chance of success)
Armour may be slotted, slottable, or simply have a good effect and receive no slot. There are also a few sets that do various things, as well as random items that have effects if equipped together. (IE: a garment and shoe when worn together give +10% max hp)
What goes in these slots?
Cards which are dropped by basically every single monster in the game. Cards are tied to a specific piece of equipment. (IE: Rocker card: +1 dex on ARMOUR, or Matyr card: +10% HP and AGI+1 on SHOES)
There are tons, and tons, and tons of ways to customize your character using these cards. Some cards may have different effects for different classes, have its effect enhanced by having a certain base stat,
or even have its effect enhanced if your gear is refined enough.
Weapons and armour (excluding accessory) can also be refined to a possible +10. For armour, it gives a boost in def, and weapons a boost in ATK.
PvP and GvG are spectacular. Requiring not only good equipment, skill setup and stat distribution, but also SKILL. Many MMO's, like WoW, take absolutely no skill to play. Better gear = better player. Not on RO. If a mediocre player has amazing, incredibly rare gear, they will still easily die to a pro of the game.
War of Emperium (WoE) is a guild vs guild event where people fight to gain control of one, or several castles. On non-official servers WoE is often highly strategic. (on official servers, it's usually just a mosh pit of skills unfortunately). Gaining control of a castle gives you access to a special guild dungeon, specific to the region of the castle. You may also upgrade the castle and employ guardians who will fight people entering your castle during WoE. Investing in a castle will also yield treasure chests full of rare items that are often useless by themselves, but can be combined to create amazing items.
*Guild investments are lost if a castle is taken during WoE, even if you gain it back before WoE ends that same day. So it is very important to keep castles to continue gaining more and more treasure every time.
The game also has cute sprites, and nearly every headgear is visually different. There are an enormous amounts of headgears, some simply for show, others simply for stats/slot/effect, and even some which look good and are good to use.
The game runs on a map system. You travel from map to map via warp points often at the edges of maps. This keeps the game world organized and is not a detracting feature in the slightest.
It would be impossible to cover every aspect of the game, there is much MUCH more to say, but i'l leave it at this.
TL:DR
RO is amazing for those looking for good PvP