My guess is thief have to be very stealthy and close range in their play. Their damage will be very high but they also die pretty fast. Their duel pistol also have very short range.
Ranger uses bow to give damage support at range but can hold their own at close encounters.
Warrior can use sniper rifle. So they will snipe mobs before finishing them off at melee range. Rifle is also useful in finishing mobs that are running away. Rifle attack rate will probably be super slow but hits hard; An extra option for warriors to play with.
Of the 3 range classes so far thief will probably be the hardest to play, just like assassins was in GW1. You can tell from the fact that the thief player died during the demonstration video. Don't expect them to tank much.