Well, like most MMOs, you'll probably find good info in class specific subforums on the game's forum. Guru is better than the official forum in this matter.
I'd say:
Warrior is a warrior. This is a standard class, but the GW2 difference is that they make sure all classes can do a multitude of things and the Warrior is not an exception. Ranged Warriors are completely viable and support Warriors are as well. You have high armor, high health and high damage if you want. One of the better designed and more refined classes in the game.
Guardian is like your Paladin type. Another familiar class type. I find them to be very easy to play early on. Greatsword feels like ez mode while leveling. You can tear through three mobs and move on. Low health, but many buffs, healing and ways to support others. A little lacking in utility skill design, but very viable and strong in spite of recent nerfs to the class.
Thief is not going to feel like a typical rogue despite sharing some similarities. Most rogues aren't as range viable as a thief, and GW2 doesn't have any perma-stealth or long term stealth skill like other games have for their rogue classes. Instead, the thief moves in and out of battle, has a lot of escapes and quick attacks, teleports and short term stealth. The initiative system also makes the thief feel different from the other classes. It's the only resource-dependent class, while the others are limited by cooldowns instead. Thiefs have very high damage but low health.
Ranger is your Ranger/Hunter class. They have pets and their pets are a good portion of their build type. Pets offer utility, control and buffs instead of just being straight tank bots. Like all classes, the Ranger can succeed at melee or at range. A very decent class overall. Haven't played too much of it, but I also don't hear much complaints about the class. ANet loved the Ranger in the first Guild Wars, so I'm not surprised to see that they made sure to get it to a nice place in the sequel.
Necromancer is a tanky, dark magic class. They currently have the most issues, due to pet reliability and broken traits. They're trying to expand the amount of viable builds for the class and improve it overall. Also, when I say tanky, I do mean tanky. Necromancers have a second lifebar as their class mechanic called Death Shroud. They can spam conditions and survive for a long time. They can also go minion master build.
Mesmer is a more unique class to the Guild Wars universe. They were once counter-casters, but in this game they are more magical rogues. Again, does good stuff at melee and at range. Though seemingly unique, its hard to say a Mesmer is anything but a reskinned minion master. You are very dependent upon your illusions to survive, do damage and just plain play the game. Half of your weapon bar skills will be illusion summons no matter the weapon you use. Mastering a Mesmer is about micromanaging the illusions and shatters of those illusions. The Mesmer does have the ability to do good damage while having good survival because of the illusions. They tossle aggro around and confuse average players.
Engineers are also unique to this game and I think they're the most fun to play. Like the Elementalists, the Engineer doesn't weapon swap but accesses more skills at once overall. This is done through weapon kits in the Engineer's utility skill slots. The Engineer is very utility skill dependent overall. What utility skills you have determines your playstyle more than your weapon choice. The trade-off is that an Engineer can play mutliple roles and do many things without having to step out from combat and alter its build.
Elementalists are the Mage-types, but are the rare kind that use all elemental forces at once. Like mentioned, the Ele doesn't use weapon swap and instead has four elemental attunements that it switches between. The ele can then perform many functions and has good damage, healing, escape and control overall. They are on the low end of the health side, but like the Guardian, they can survive pretty well in spite of this.
