I thought it might be a good idea to compile a list of all the things we currently know about the classes so far. If you find any information, feel free to add it.
Titan: Pretty much described as the typical soldier, and has even been compared to Master Chief a couple times. I believe that means we can assume s/he's the "tank" type character. Also has been mentioned that he uses heavy weapons, perhaps that's something the class focuses on. Anybody know what kind of special ability, if any, the class might have?
Hunter: Described as the "recon" or "sniper" class. I'm not sure how one manages recon in a game like this- perhaps the character will have some sort of cloaking ability? In the E3 gameplay demonstration it was shown to activate some sort of fire ability on its weapon, and did something similar in one of the cinematic trailers. Special ability?
Warlock: The magic class. So far the character has shown the ability to fire and detonate some sort of arcane explosion from its hand. In concept art it was shown using some sort of fire spell, and in an interview it was said they could "conjure and throw fireballs." Whether these abilities are still in the game is currently unknown.
Vanguard: Absolutely nothing is known about this class. It was mentioned in an article previewing the game, and could potentially have been a typo of some sort.
I should also mention that, in the article that mentions the Vanguard, there is mention that all Guardians utilize some of the "space-magic" given by the Traveler. It's possible that, like the Warlock, the other classes will receive supernatural abilities that differ per class.
Seems to be a lack of specific info in general. I figured we would have gotten more details by now. I will say that they seemed to show off a lot of the Warlock class in that gameplay trailer. I heard that the duster was a signature clothing of the Warlock, as the cloak/hood and heavy armor are specific to the Hunter and Titan respectively.
I'd also like to see some weapon info. What types of weapons are we gonna see? Assault rifles, snipers, rocket launchers, pistols, SMGs? Are weapons specific to each class or are each class just better with certain types of weapons via perks/enhancements ala Borderlands 2. I'm surprised we didn't get more info after E3 honestly.
Some of the Warlock's abilities have been revealed! Apparently they're: Solar Flare: Conjure the power of the sun and release it like a grenade. Nova Bomb: Channel some of the Traveler's mysterious forces and release it in an explosive area-of-effect attack. (Likely the ability the Warlock has been seen using in the E3 gameplay trailer.) Glide: Summons the forces around you to increase and sustain elevation after jumping. (Likely also seen in the E3 gameplay trailer.)
IGN recently released a gameplay video. It's much like the E3 gameplay video, but I saw a couple things that may pertain.
The Titan class seems to have some sort of AoE, electric ground-pound attack.
Also, during the encounter with the Fallen, when the devs are fighting the spider-robot, at one point in time you can see one of the Hunter players execute what looks like a double-jump. Since we know the Warlock class is capable of using a Glide ability after jumping, if the Hunter is capable of using a double-jump it's possible that all classes have some sort of movement/jump ability. This is purely speculation at this point, though, and the double-jump may have been unrelated to the class.