The Agg/Def bar is a nice concept. Basically, the intent is to shift between a defensive position and an offensive one. Full Def (Agg/Def bar all the way to the left), will make it so that you hit slower and cast slower, but are much harder to hit. Full Agg (Agg/Def bar all the way to the right) will allow you to hit faster, cast faster and also be more likely to be hit.

At most times, Full Agg is where you'll be. The goal however, is to be the farthest you can to the left side of the bar.

This may seem like a confusing statement since you can put the slider wherever you want...

The other thing you want is to be hitting as fast as possible (1/1, or 1 sec to attack, 1 second to recharge). As you get more Init in your weapon type, it will be easier to hit 1/1. For exact info on this, find out which of your weapons is slowest, lookit up at aodb.info, then go to "Simulate Weapon Damage" and find out what the minimum init required to hit 1/1 is.