This is a complex game to review. It has many different elements that appeal to different types of people, but if you're one of those whose tastes fall right in the center, you will absolutely love it.
The world is an amalgamation of conspiracy theories and horror themes. It's a very bleak world and this is very much apparent once you're out of the three primary cities (London, New York, and Seoul), which are very detailed and well-constructed (London being my favorite by far). If you're a fan of Stephen King, HP Lovecraft, or Supernatural, you will probably enjoy the atmosphere. It will be even more enjoyable if you collect and read the hundreds (thousands?) of lore items scattered throughout the world that offer greater insight into specific events or groups.
There are three factions, the Dragon, the Illuminati, and the Templars. They are united in their fight against the darkness, but often very much at odds over historical events. They each have their own storyline, and many of the responses to different quests are unique to the faction. The Dragon have a more eastern/philosophical outlook on things, the Templars are a bit more mystical and militaristic, and the Illuminati are very business-like and technical. The stories you encounter reflect this, as the Templar story often feels very mystical while the Illuminati story has more of a conspiracy feel to it.
The dialogue is particularly entertaining. You do not get to make responses, as in SWTOR, but I found the NPCs much more interesting to listen to in TSW. They often seem like real people and make amusing or enlightening analogies. You can talk to many about a variety of topics that do not directly apply to a quest, and I've spent several hours doing so without realizing how much time had passed.
The mechanics initially seem to be typical of MMOs, but differences become more apparent the further you get into the game. Instead of each quest being about killing 10 goblins, quests can have you trying to investigate the origins of the town, setting up defenses and fighting off attacks, or tracking down and stopping a supernatural killer (and killing 10 draug...). If the quest is fairly straightforward (kill 10 draug), you will see markers on your map and on your screen indicating where to go. However, if you're expected to figure something out this is often not the case. For instance, you may be given a vague idea of what you're looking for and through the use of google (there's an in-game browser and a website for various groups in the game, such as the Kingsmouth newspaper) and in-game clues (hidden in or behind paintings, in architecture, or just about anywhere you can think of), you're expected to figure out where you need to go and get there. Once you've arrived, the quest progresses to the next stage.
You're allowed to equip 7 active and 7 passive skills of hundreds purchasable using rewards for gaining experience. Some build resources, some use them. Some set certain states on the enemy (weakened, hindered, etc.), while others will exploit those states. You mix and match the 7 active and 7 passive skills to create a "build" that you enjoy and feel is effective. This can be done on the fly as long as you're out of combat (lose a fight? swap skills and try again). The states can also be exploited by other players, so you could theoretically get with friends and work together to create builds where you exploit each others' actions to greater effect.
As for the combat, again, the mechanics initially seem typical button-mashing. The states and skills mentioned in the previous paragraph add greater variety, but there are also indicators that appear when an enemy is about to use a particularly punishing attack that allow you the opportunity to either run out of the way to avoid being hit or to use an active dodge button for the same purpose. Although avoiding these attacks can be helpful in solo-play, it becomes imperative to avoid as many as possible in instances (dungeons) where one such hit may well kill you or put anyone healing on their toes to try to keep everyone alive. Fortunately, you do not sacrifice much damage or many actions by avoiding attacks, as all combat skills are usable while moving (and many situations will leave you dead if you do not use them while moving).
Armor is in the form of talismans, whether they're rings or belts or other items that do not alter your character's appearance. Clothing is separate and only alters appearance. There is a large clothing store in London where you can customize your character after you've earned some money.
Although there are many servers ("dimensions"), you can play with friends on any of them. If you join a group with someone from another server, you will be asked if you want to be transported to their dimension. When you leave the group, you are returned to your own. You cannot PVP on any server but your own.
PVP is restricted to one of two battlegrounds and one warzone. The battlegrounds only allow around 10 people per faction. The warzone allows 75 per faction and is persistent. Controlling various areas in the warzone provides a server-wide buff to the controlling faction (so people in dungeons or just out questing benefit). The power of the buff depends on the number of areas your faction controls.
There are still bugs (as there always will be in games), but generally you can either skip them or ask for an invite to a server where a particular quest isn't bugged. There aren't too many, but the ones that are there can be frustrating because it's often difficult to know whether or not you are stuck because of a bug or stuck because you just haven't figured something out. Except in a few cases (which often pop up in the help chat), most of the time it's the latter. Fortunately Funcom seems to be working quickly to fix the quests that are actually bugged, and reporting it to a GM will get them to bump the quest so that the bugged part is completed.
If you like solving puzzles, learning about the background of the game world, or building "decks" out of hundreds of different skills, this is probably the game for you. If you're looking for something light that you can breeze through without much thought (and there are days when that's the sort of thing I want), you're better off looking elsewhere.