The main difference was abilities (about 4 in DA:O iirc, more in Awakening) that directly influenced threat-ratings, thus making enemies choose specific characters over others as targets. The gameplay idea is that you can in this way direct enemies to attack characters that can more easily shake off the attacks (the tanks) rather than the weaker ones.
The bigger issue is that, if you have no way to control which enemies attack what character, that means you are encouraged to make characters with similar ability to soak damage. And this takes away from the variance of characters design, as you encouraged to design characters the same way, at least in this aspect.
I can think of 3 ways to solve this:
1) Make stupid AI, that attacks the first enemy it sees, so players can solve this with easy positioning. I think the IE games had a lot of stupid AI like this, but there was several exceptions as well, even within a specific game I think.
2) Make combatant positioning count so that enemies can't attack any characters they see freely. The attacks of opportunity used NWN is one example, though I don't think it worked at all (it works fine in PnP).
3) Give the player abilities to influence what enemies attack directly. I'm not sure if WoW was the first one to do this, but it's probably the most famous.