Elementalist : This is typical caster type with different types of elements (fire, water, air and earth). Since they have biggest amount of spells at one moment, some consider they are hardest to master. So if you like harder challenge this might be class for you.
Warrior : This is straight forward dmg dealing class that also have strong tanking options. Their special ability is their big amount of weapons they can use. So you can make many different builds. Many people say this is easiest class to play, but also easiest class to counter.
Thief : This is typical assassin type of class. They got stealth, many skills to stab from behind. Their special ability is stealing which gives extra buffs and other things depending on your build. However they made this class very different compared to usually assassin. Momo played it during beta and it was tricky to say at least.
Guardian : This is typical tanking class but also have powerful offensive options. You can spec them to be insanely tanky, but also you can build them to be offensive power. Their special ability are virtues that give very strong buffs.
Ranger : If you like to have pet, and huge range this is class for you. Their special ability are pets, and also they are good at controling opponents with slows, and condition dmg. They can have big dmg also.
Necromancer : This is another mage type of class. Their special ability is shroud mode that makes them insanely tanky and dangerous. They are masters of death arts and can raise many minions to aid you.
Mesmer : This is another mage class, but their special ability are illusions and phantasms. You can have 3 illusions up at the same time and you can destroy them for effects. Mesmer is complicated class in my opinion and I would not recommend it to new players.
Engineer : This is new type of class, I never saw anything like this in any other mmo. They have turrets, bombs, elixirs as their skills. If you wanna try something fresh, something u never saw before this might be class for you.