[color=red] WARNING: POTENTIAL GAME TRIGGERS [/color]
I've looked through the "Games Reported as Being Addictive" list and noticed only one game by Nexon on there: Combat Arms, a FPS. I think the list should also mention MMOs they have that I know for a fact have people hooked, especially since virtually all of them are free-to-play.
There are two main Nexon games I specifically request to be listed on this site as a warning: MapleStory and Mabinogi, their top two MMORPGs in the NA branch.
Many have heard of MapleStory. Nexon's even advertised it on American tv at one point. It's Nexon's most popular game and well-known game here in America; a lot of people that haven't heard of Nexon much before know it as "that MapleStory company." MapleStory is a 2D sidescrolling MMORPG advertised as "quirky and unique." You have the typical grinding, leveling, exploring, and questing of any MMO on there, just with a non-traditional view (and though "cutesy," the pixel art's not bad, I must admit). I tried this game at one point and wasn't into it, but it does have real MMORPG qualities that get people actively playing, and it brags about having millions of players as well. Though MapleStory is the most popular game, it's not the most immersive, and I'd mostly warn against it as being a sort of "gateway drug" into Nexon's free-to-play library of games: once you get bored of Maple, you check out what other fun and FREE quality games Nexon has.
Mabinogi is the other game I'd like to report. It makes me feel bad doing so (since it's my own personal offender and as an addict I love it so much), but the cold, hard truth is that it has an iron grasp on most long-term players of it. Mabinogi is a 3D MMORPG advertised as a "Fantasy Life," and it really does work as a replacement for your real one if you let it, unlike Maple. A lot of people have described it as a graphically-superior and larger, better version of RuneScape (which I've never played personally). But from what I know of it, you can play Mabi--and get addicted to it--in more than one way.
I think of Mabi as a hybrid of competitive, skill-based games like WoW and of community-oriented, social games like Second Life, both of which are major offenders of people on this site (and I whenever I read a forum post on either one of these, I strongly relate with my Mabi experience). This makes this MMORPG extra "dangerous" because you can get addicted in more than one way, depending on your play style.
In a way, the leveling system in Mabi is even more addictive than most other MMORPGs because it's unique: there is ultimately no cap, and therefore no real reason to stop. There's a rebirth system where you can rebirth for free after three weeks (after one week if you pay because yes, it's one of those pay-to-get-ahead games), which resets your "current" level back to 1, but keeps a cumulative level that has no known limit. Pretty much in NA if you're over 10,000 total level you're considered "pro," but there will always be people rebirthing and levelling if you leave, so to stay on top most people make it a routine or "chore" to grind up to a decent current level for the week (the "current level" cap is 200, but most people just get to lvl.100 before they rebirth because getting to lvl.200 is insanely tedious) just to stay adequate in the game. For each level you gain you also get ability points to rank skills, and since some high-end skills take up to 100 AP per rank, and there are a LOT of skills, there's always a reason to keep squeezing in play-time to get ahead in some way. This, of course, is just one way to play the game.
There's also the social scene similar to SL. It's weird when people say that friends you make via the game aren't real, because in Mabi they really can end up feeling like family for some. I'm thinking this is because the game is so relaxed and has a non-action-oriented side; you don't always have to be out killing things, you can just sit down and play music for your friends by a campfire in the square and have an awesome time. Also, there are no distinct classes, so you can pretty much be and dress like whatever you want. Your character is just like an "avie."
This game is most effective on long-term players. Pretty much, the addicted ones are those that have been playing for years (since the year the game was released in NA) and just haven't left their Fantasy Life. Unfortunately, the game has become such a huge part of their "real" life for so many that I suspect a lot of them are in heavy denial... But that's just from my observation. They may feel happy playing every day (and believe that they choose to play because that's what they want to do with their lives), but many of the real-life stories of my friends in-game are very, very sad, and I see them logging in every day. I just can't reach them, so I'm hoping that any of them that have the slightest notion to quit and then one day find this site can realize that yes, there are some of us Mabinogians that want to quit but struggle with it just like any gaming addiction.
Live your story.