It is hard to give any constructive advice when I do not know how the AI works (and I do not even want to know it, that would be big spoiler and chance to exploit). I do not think that the AI is broken, but it is certainly different to what we knew and exploited so easily.
Rogues are fine, they have plenty of options to survive, and with a bit of tumble, they can even roll around the fighters who can AoO the silly enemy to death. But when a wizard, who is not prepared for battle (using shonmann's example of the innocent bystander) stands still and he spots a hostile creature (who he cannot kill with a single spell, probably cannot even stop for a second) and the said creature ignores everything and everyone in his way to take the wizard out…well, the only choice you have to make is to use potion, wand or spell of invisibility. It happens way too often.
Perhaps a check could be added to see if that squishy person is just a commoner in rags or actually casted a spell that I have seen with my own eyes?
If the wizard is in a quest or in battle, and one or more creatures break the line to go for him, I do not like it but it is acceptable, as many of you pointed out that is what most people do in a quest anyway. Ogre Berserkers, surrounded by 3-4 badass fighters, effectively cornering him, tend to ignore the melee threat and switch to throwing axes to kill the wizard in the back. It's like "I know I cannot win, but I will take one with me to Ogre Valhalla".
Perhaps I was lucky so far, but every quest I have been in the past two months only stealth chars or skirmishers went for the wizards, the fighters would not run past the elite bandits or orc veterans just to engage the bandit mage (or even elite mage). Flankers kept the casters busy (they tend to run away invisible), and when the melee opponent was dead, the fighters joined us killing the arcanist.
I am unsure if it is feasible or not, but:
- How about archer type of enemies target the unarmored/light armored characters first, run away from melee or switch to melee weapons if necessary
A tanglefoot bag is a nasty surprise as well.
- Melee enemies target the melee characters as a priority, and if there are more enemies than PCs, some would switch to unarmored/light armored characters.
- Stealth based characters with sneak attack could try to sneak upon the mages / support characters as an opening move (same as PCs do :) ).
- Make it a ~75% chance to do these, and and 25% to do something unexpected...like, breaking lines and chase down someone in the back with little to no regard to his own safety.