Whizbang Dustyboots said:
Likewise, enemies have gotten smarter. MMORPGs don't have dumb enemies due to a limitation of the engine -- knowing to attack the healer, or whatever, is pretty obvious and it's actually coded into the NPC response system of even EQ1. It's just, as Blizzard discussed during BlizzCon, it might seem more realistic for Onyxia the black dragon to kill all the healers first, but who the heck wants to play a healer in that fight after that?
The problem is that monster stats are pumped up because they're so stupid. So of course making them "smarter" is going to be a problem if they retain the stats designed to make them challenging (to some characters) when they're stupid. People still play healers in PvP, even though they're priority targets - the best form of Taunt in PvP is casting heals

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Tanking is basically fighter work, although other characters can also do it. The idea of the tank is that monsters can be manipulated into attacking the character with the best defense to protect the rest of the group. Sword and board characters are basically tanks.
Then there's damage dealing. Sneak attacking rogues, fireballing wizards, power attacking barbarians, etc. As in DnD, some situations in MMOs tend to favor some types of damage over others, but in general you can substitute one type for another.
Healing is far more critical in most MMOs than in DnD. Healing is generally far more efficient than comparable offense, so it's even more dominant. And most encounters are designed to "require" (there are often workarounds, but not easy ones) fairly steady healing. In DnD, the cleric might patch people up after the fight, possibly making an emergency save during it. But he's got other good spells to use, and can waylay people with his mace effectively. In most MMOs, healers are generally more dedicated - other stuff usually needs to be worked in carefully.
Crowd control is designed to reduce the number of foes the group must face at once. In DnD terms, you'd use battlefield control spells like Wall of Force, Entangle to limit enemy movement, Hold type spells, or powerful debuffs.
Then there's utility, which covers everything else. Scouting, teleporting the group around, etc.