Not perfect, but unique and free!
First let me say, its a repetitive game. You kill monsters, and thats about it. 95% of the quests you get are to kill specific monsters for a quota or to collect drops, 4% of the quests require you to go to an area and collect things that spawn on the ground, and 1% of the quests require you to deliver things from one NPC to another. If you want any kind of plot or story arc of any kind, this game has none. but I will mention a few things that save rappelz from being ridiculously boring.
one of them is the pet system. pets are a great aid to the game because you can outfit your pet with weapons to up its stats enough to actually be like a secondary party member that you control. this makes it much easier to combat monsters that are higher level than you. also some pets like orcs and yetis can tank for you if you solo a dungeon, and blue fairy pets can heal for you if you solo a dungeon. so pets are a neat addition to the game, as you dont always have to go it alone while killing monsters.
another thing that i didnt see mentioned before is the dungeon system. there are always dungeon parties being formed in any city in any server, its very easy to join one if you fill a dungeon party position (healer, tank, etc). dungeons offer increased money drops, item drops, etc, and its fun to team up and take down a bunch of high level mobs, leveling quickly and getting mad loot. plus the low level dungeons are very accessible, you only need to be lvl 30+ to join a low level dp. you can get to lvl 30 in about 2-3 days of moderate time commitment.
the rappelz community in general is mature and not annoying, which is a big pull if you just want to play the game without having to deal with scammers and such. as far as character customization, there is very little but there is a lot of depth to the item customizations and even the style of weaponry you decide to use. i enjoy this game, and i know its not for everyone but if you are looking for a free mmo, this one looks good and has unique features that set it apart from the rest.