All the classes in Diablo universe:
The combined classes are as follows:
Witch Doctor = Druid + Necromancer
Barbarian = Warrior + Barbarian
Wizard = Sorcerer + Amazon
Monk = Assassin + Paladin
Leaving us with...
So, the Bard wasn't really a class, but whatever. So the Bard has the ability to identify items, use magic, and dual wield; while the Monk(Hellfire) has a natural resistance to pain, and can use a skill called Search.
There is also a recurring theme of ethnicity and location:
Barbarian = Caucasian; Mountainous, Grass
Witch Doctor = African; Dead, Plains
Wizard = Asian; Ruins, Valley (Dungeons)
Monk = European; Desert, Ruins
So, to be different from the other four classes, I believe the class will be as follows:
It is a male, from a location that is cold/winter (Dreadlands). This class is sort of like and Eskimo if you will. When we talk skill wise, it must be good at Searching, Natural Resistances (to the environment/cold), good at identifying items, and use dual weldable weapons. Also, Blizzard has made a point of the Runes being an important part of their game, so I assume that this class will have some connection to runes (can make them, perhaps?)
The actual class itself, I think will be a Hunter/Survivalist, someone who excels at taking out enemies by using the environment, and using tactics. So far we haven't seen any aspect of stealth play (Lock picking) so I'm sure they won't leave out that aspect in their game. Another thing is shapeshifitng, which was a fun part of Diablo. This class will be quick at defeating enemies one at a time and be able to buff friends using incantations or something along the lines of a buff.
This is just my guess, I would love your thoughts on the fifth and final class.
(NOTE: I didn't put Monk underneath Monk because the Monk from Hellfire was different from Diablo 3)