First off, while The Secret World is set in our real world, it supposes that every myth and legend you have ever heard is true. Bigfoot, zombies, Hollow Earth, ghosts, werewolves, vampires, killer tomatoes; all of these things can be found in TSW.
There are three playable factions in the game: the Dragon (headquarters: Seoul, South Korea), Illuminati (headquarters: New York, USA) and the Templars (headquarters: London, UK). The Dragon believe in chaos and silent manipulation; the Illuminati favour corporate power and technological advancement; while finally the Templars see themselves as the old guard and the protectors of the ancient ways. Although you can group up in PvE content with members of other factions (this is a nice little feature), for PvP it is strictly your faction versus the other two.
There are no classes and no levels in The Secret World. Instead, you "build" your characters abilities from over 500 that will be in the game at release. These can be acquired by doing missions, helping your faction, perhaps even through PvP as well. There are seven active ability slots and seven passives; combined, these fourteen abilities constitute a "deck" and this is where Funcom say that the true strength of the game lies, in acquiring and building new decks to be able to truly create your own characters from the literally thousands of combinations available.
Gear in The Secret World is purely aesthetic. No more keeping pieces of equipment that look terrible but have amazing stats, you can instead make your character look exactly how you want them to look without needing to worry about it affecting your skills. Other MMOs have tried experimenting with such systems (notably Rift, which lets you customise a "wardrobe" set of gear that controls how you look to other people, without affecting any of your stats and skills) but this seems to be the most liberal system yet when it comes to equipment and visual style.
One such example of these "investigative" quests is when you are tasked to investigate an old murder case, with no clues or hints on where to go or what do. You have to go to City Hall in the relevant area and dig up an old report about the murder which names the suspect. You then have to go to the police station and use their computer to find some information about the suspect, in this case what cell he was housed in. Next you visit the cell to look for clues, but find absolutely nothing. It turns out you have to actually kill yourself and then run to the cell in ghost form in order to find the spirit of the murderer and bring closure to that particular investigation. Not exactly your everyday "kill ten rats" kind of quest.
Funcom have stated that they actually require use of search engines (i.e. Google) so much that they will be including an in-game web browser, so that you can browse the web without needing to tab out of the game. It's only going to be a reskinned version of Google Chrome but hey, anything to aid in the immersion is fine by me.
Unfortunately there isn't much information available about PvP in The Secret World, other than that it will exist in both an open-world PvP area and in smaller "battleground"-type instances that have come to represent what most people think of when they envision PvP these days.
I am very excited about The Secret World. I think that a real-world MMO is something that could be truly groundbreaking, and meshing it with a fantasy theme could really make something that is truly special. No classes and no levels are also two things which are going to get "oldschool" RPG enthusiasts interested (former Ultima Online players, for example), and I particularly like that gear is not linked to your stats, so you are able to wear what you want, when you want.
On the other hand...well it's Funcom, the developer that gave you the ultimate bait-and-switch with Age of Conan, which was amazing for the first twenty levels in Tortage and then just amazingly average for the rest of the game (though it is apparently in a much more polished state these days, following its free-to-play conversion and several patches). Funcom have also said that TSW will follow both the traditional subscription model as well as including a cash shop, which does not sit well for some people as they prefer either one or the other. It remains to be seen how well they can combine these two traditionally disparate models together.
Will The Secret World represent a paradigm shift in MMOs, or will it instead fail to deliver and just give us more of what is already out there? It shouldn't be a secret for much longer.