Follow-up: The Secret World's weird and obscure substitute for character class
If you've played any role-playing computer game since Diablo II, you're probably familiar with the idea of "skill trees." In any other game, your character class has (depending on the game) a certain number of skills that you learn as you level up, replacing earlier skills with better ones or just adding to your powers. In general, you have to either pick them in order, or they unlock farther up the tree as you go up in level.
The Secret World has skill trees, too, although they use a weird UI to disguise this fact. 18 of them at first, each with 9 powers, but every time you fill out two related ones, it unlocks another six skill trees, each of which has another 9 powers. Oh, and there are 3 more completely unrelated "universal" skill trees, with another 9 powers each: a grand total of 525 powers that you have to unlock by spending "ability points" that you earn every so many thousand XP. You keep earning them; once you fill out your skill trees, you can start filling up other skill trees ... and you do want to, for a reason I'll get to in a minute.
Those initial 18 skill trees are tied to the 9 weapons of The Secret World, each of which has two trees; if you absolutely insist on thinking in terms of character classes, I won't stop you. Each of the weapons has a DPS tree. All 9 of them also have an "other" tree: hammer, blade, and chaos magic are your tanker weapons; claws, assault rifle, and blood magic are your healer weapons; and pistols, shotgun, and elemental magic are your crowd control, buff, and debuff weapons.
(There is no stealther weapon, no weapon that grants you stealth and assassination abilities. There also is no summoner-class weapon per se; shotgun, pistol, and elementalism eventually get turret-summoning or pet-summoning abilities, but very minor ones that aren't at all class-defining. Tank, heal, and buff/debuff are your only three non-DPS skill tree types. I'm slightly disappointed; I usually prefer playing the summoner or assassin classes. Maybe some day we can talk them into adding sniper rifle and summoner fetish weapons and weapon skill trees?)
From all of the click-power abilities from both of the basic trees for your two equipped weapons (and eventually from all 12 of their advanced trees), call it your choice of roughly 20 basic and 60 advanced click powers, you choose exactly 7 to be on your active toolbar at any time: any mix of DPS abilities, the secondary abilities for the one weapon, and the secondary abilities for the other weapon. So you can, if you want, build for pure tanking (say, just the secondary trees for any two of hammer, blade, or chaos), or pure healer/support (just the secondary trees for any two of the other six weapons), or pure DPS (just the primary trees for any two weapons). You absolutely can do that, and then play it more or less like any other MMO. But you don't have to. If you are ever going to play solo, you don't even want to; you will want a solo build of some mix of DPS and other, because a pure DPS build will get you killed by the first boss or the first zergling rush of minions.
Note, by the way, that you can level up all 525 powers, eventually, and store 5 or more "builds" of selected powers, weapons, and talismans (buff/armor items that don't show up on your character), and switch between stored builds in a couple of button-clicks any time you're out of combat. There is no reason you can't spec a tanker build, a support build, and a DPS build just by fully training up at most 4 of the 9 weapons. And because of the way the XP system works, once you fully train up 2 of the starter trees, training any more starter trees can be done practically overnight.
Now the super-complicated part, and the reason why you will eventually want to train all 525 powers:
I told you how to pick your 7 primary powers, your 7 active powers, your 7 click powers. Now the passives.
See, here's the first thing to understand about the advanced skill trees: for almost any weapon, you can eventually find an attack that is almost any possible combination of plain single target or single target channeled or rectangular AoE or circular AoE or cone AoE or ground-targeted AoE or infectious multi-target; plus charge-up builder or charge-up spender; plus aggravates or confuses or buffs or heals or debuffs or slows or immobilizes or knocks down or stuns or increases crit chance or armor piercing or applies DoT or just a couple of percent stronger. That's why there are so many active skills. The reason they can say that it's level-less, or mostly so, is that the more advanced skills don't do any more damage, they just offer a wider variety of combinations.
Scattered across all 75 skill trees are a total of roughly 300 passive skills. Almost all of them are agnostic of what weapons you actually have equipped; for nearly all passives, if you've trained it, you can use it with any combination of weapons. What most of those 300 available passives does is cause something to happen when some combination of attack type and special effect type happens. You can pick any 7 of those passives to be active at any given time, and save that with your builds, too.
So not only do you have nearly unlimited ability to multi-class or single-class to your heart's content, and not only can you level up alternate builds at an astronomical speed once you level up your first build, but you have this weird "deck builder" like system of picking 7 cards from a potentially huge deck to do things like "if the attack slows the target, apply a damage over time effect, and if the target has a damage over time effect, buff the player, and if the player gets buffed, spend all charge-ups to cause a small explosion around them that injures enemies when the buff wears off." That's the obscenely complicated and tricky part.
A note about character appearance: your two equipped weapons show up on your character at all times, and they get cooler looking as they get higher level and/or more rare. (Blood magic is a gigantic grimoire strapped to your back at all times, chaos magic is a fetish mask or small shield ditto, and elementalism is a small fetish hung from your belt; the others are, well, weapons worn where you'd expect them to be worn or in your hands.) You can only equip one tier 9 active ability and one tier 9 passive ability; several of them wrap your character in a visible aura. You get your buffs and armor stats from a collection of 9 invisible "talismans" carried in your pockets or otherwise not shown.
Everything else is up to you; ordinary clothes are relatively inexpensive, with the shops in the publicly-accessible part of London having the best selection. As you level up, you also unlock a series of faction-specific uniform pieces that are entirely optional; some whole-character uniforms, some mix-and-match pieces. So other than your two weapons, and maybe some aura around you, you can dress however you like most of the time.
(PvP is the one exception; when you queue for a PvP zone, it makes you pick from one of your faction's three PvP uniforms, each of which gives either a tanking or DPS or healing buff to you. It makes it easy to see at a glance who's on which side, which matters in big fights.)