Hexen's class system made it unique compared to other shooters of the time. It gave the player another meaningful choice to make, whether that was for the single player story, playing in co-op, or taking down your friends in deathmatch. In Doom, everyone was the same, player skill aside, and that could get old (after a very long time, admittedly). Playing the game as a fighter, then a mage, then a cleric tripled the replay factor. In co-op mode, you could tailor your character to the needs of the group. Any combination of players worked pretty well, the more variety, the better. Players had cover one another's weaknesses, which only added to the sense of teamwork. I always preferred to play as the fighter since I wasn't exactly good at dodging, and the extra health and armor really helped. A nice mage partner and I could do quite well together.
Hexen was followed by a sequel, Hexen II, in 1997. Based on the Quake engine, Hexen II was one of the first games I remember that truly impressed me when running on a machine with a good video card. Hexen II revamped the classes, and added in another for a total of four. It also incorporated even more core RPG structures, like a leveling system, with stat bonuses and extra hit point and mana pools rewarding advancement.
The original Hexen was ported to several home consoles, including the Nintendo 64, Saturn, and the original Playstation. A version of Hexen II is in development for the iPhone. Both games, as well as Heretic, are available on Steam, but being as old as they are, it's uncertain whether online multiplayer would work or not. I don't know about you, but I'd buy on day one if Hexen or Hexen II were released on Xbox Live Arcade or Playstation Network!