[Edit 7/4/2014] Review has been slightly revised prior to the "New Beginnings" patch. The most noteworthy bits of information: Rebirths are now free and can be performed every day for newbies, and weekly for experienced players, and beginner benefits now extend till level 1000, thus making the game even better since all players don't have a huge waiting time to continue playing the game.
As someone who spent way more than 3,000 hours on this game, I feel as I have seen everything this game has to offer, and I still enjoy coming back to it. Although Mabinogi is a free-to-play game, don't let that label drive you away from trying this unique game out.
Mabinogi is about the story of a random Milletian (You) who was sent to the world of Erinn, one thing leads to another and you end up helping to save the world from one of many dangers. The graphics are cell-shaded and provide an anime-like feeling, the background and world is inspired from Celtic Mythology and fantasy titles, and for what it is, realistic looking graphics wouldn't make it look better. I should mention that the game has minimal optimization settings since it's a title from around 2003, but most computers should be able to run it and handle it smoothly.
The core gameplay of Mabinogi is the freedom to develop your Milletian to be anything and everything. Most of the skill sets are well balanced and influence other skill sets or play styles, giving players the ability to fight as stylish or brute-like as they please while being efficient and reliant on the player's actual observatory skill and reaction time. Skills aren't trained by just leveling up, you have to actually use them in specific ways to be able to train them, and at the end of that, you can advance them to a higher rank, which rewards more damage output and stat increases that carry over no matter what you decide to do next. Combat is also not just clicking and letting your character do all the work. Player input helps determine how the combat pans out, and goes in deeper than just mashing hotkeys. The game is tailored by "If something is bigger, than it is stronger than it as well." It's also a challenge to fend off more than two enemies at once at first glance. Once a player gets in the swing of things and understands what skills are effective and when, then yes it's easier to play and interpret what an enemy will do next, and what skills they should look into advancing.
If you are already pretty beefy, there are a plethera of life skills which people can work on that reward the ability to produce products of the sort and permanent stat bonuses. Most people only do this on their downtime or till after the point where they feel they are strong enough for what they run.
The other core function is the ability to "Rebirth." This resets your current player and exploration level to 1, giving you the chance to level-up as easily as you did back when you were level 1. Which in turn, gives you more AP to spend on skills. This means there are no character level caps, and you can keep going to learn new abilities and skill sets. Rebirths are available for free every day for players under total level 1,000, or every 6 days for players over that 1,000 level mark. Some people dedicate a rebirth and pool all the AP they recieve in that timeframe towards a certain skill to get that mastered.
Mabinogi has a social aspect as well. A large chunk of players in Mabinogi are either anime fans or people who just want to hang out, and most of these people actually play the game as well. There are various sub-groups people are "put in" based on what they do (PvPers kill each other, Fashionogi stand around flaunting their looks, Vendors/Sellers sell things or advertise, etc.) and a lot of people connect more based off that "system." The forums on the site are also well active, with many people who communicate amongst one another, including with some of the more well-known Nexon employees or community managers.
The premium shop has it's fair share of criticism since Mabinogi is a free-to-play game. However, in my personal experience, It's not that often where I will say something isn't worth the money. Ranging in the premium shops are pets and mounts which serve as inventory space or... obviously mounts to ride or fly from place-to-place with, the premium services (which isn't required to play the game in it's entirety), and various other goodies. It boils down to if you believe it's a reasonable price for the item and if you are willing to pay for it. Along with this, I'll get criticism for saying this as well, but Nexon is pretty generous with their premium content. A lot of events and even some in-game quests reward players with premium content, double training / AP events, and more content to fondle with.
Mabinogi is a truly unique game overflowing with content that you can choose to aspire to or not. When you sit down to the game and play it for the first short while, you will feel accomplished just starting out and getting the hang of combat and gathering. With that feeling, you'll want to get better at the game and pursue playing it more. At first, it's a steep learning curve, but don't let that concern you. With practice, experience, and time, you will be able to take on anything, and you will fully enjoy this game.
If you have any questions regarding features or anything else in the game, Feel free to note my in-game character, Aradefaktare of Alexina server.