The only revolutionary aspect of the game is the class system. Even then, it's taking Guild Wars' feel for having a custome class to a new level.
However, not all custome classes work well. Especially with mages. A stormcaller with Pyromancer is a very bad choice. Stormcaller is Water and Air magic, Pyromancer is Fire magic. The mechanics of the two mage classes also pidgeon hole you into using one or the others' offensive magic spells. Casting Air and Fire spells in combination will make you very very innefficient. As a result, pretty much every mage class takes elementalist as a secondary as its talents are genertic enough and you get a pet.
I suppose that's the draw back of the customization of classes. You only pick your secondary class not because you want to play that class as well, but because the passive bonuses support your main class well, or you gain a certain ability at the start that's useful.
I've been playing about with class builds. I've only found two combinations that you really do make use of the abilies of the secondary specc.
Marksmanship/Assasination (Poison, stealth and dealing bleed damage when you crit).
Chloromancer/Warlock (Go some distance into Warlock to gain the spell that turns health into mana. This works well to solve the Chloromancer's mana issues. It also great as the Chloromancer is a healer-dps. Like shadowpriest in WoW. The health loss to gain mana is easily recovered).
The game does suggest recommended classes to accompany your main one, but it is best to "browse" the other ones to find the one that suites you. Also use trial and error. On my bard, I chose a soul that'd buff my damage. But then later on I realised I'd be spending alot of time using healing/supportive abilties, so having better suriviability was better than deal a smidgen more damage.
You only need 4 characters at most. Though you get your first 3 souls early on in the game, you can unlock the other 5 later on via quests (above level 12).
You get 4 "roles" which are like the speccs in WoW. You can pick a role from any of the 3 classes you have available. 4 roles is enough.
1 Tank role
1 Support role
1 Ranged DPS role
1 Melee DPS role
The only classes that can't have all 4 is Mage and Warrior. So if you like a handful of different rogue types. Like I enjoy Marksmanship, Bard, and Assasination. Save up to unlock a total of 3 roles, specc each one to the desired class and just switch between them upon whim when leveling up/pvping/rifting what ever. You have no excuse to be bored inless you genuinly hate your class' core mechanics.
That is what I have also noticed about the classes. Each of the 4 classes has a distinct mechanic.
Mage - Mana and a Charge Bar. The charge bar fills up when casting, and can be used when using certain abilities. Chloromancer can buff their damage while draining it. Pyromancer can use it to gain the next X spells to have a significant mana cost decrease.
Rogue - Energy and Combo points. Eneregy constantly regenerates and you use combo points to use finishers.
Warrior - Energy and a kind of Combo point. Unlike the rogue, the combo points appear on yourself, and it's only 3 at the most. They're spent on finishers as well.
Cleric - Mana and cooldowns. I've only played the melee and tank clerics (aswell as cabalist). But, they rely on abilties having cooldowns and being instant cast. If you enjoy priority queues for abilities, you'll love playing a cleric.
As stated earlier, almost all the classes can have a Tank/Melee DPS/Ranged DPS/Healer role. So, pick the class with the mechanic you like, and try to malform your class into the role you want. Mages have no tank or melee dps speccs (officially). Warriors have no healer and ranged dps speccs (officially).
The problem with having this much freedom on what you want to be, some very freaky builds can appear that are game breaking. So, if you come across a build that's ZOMG AMAZING, don't be too disgruntled if you find it getting nerfed.