I play a tank and try my best to keep aggro on me at all times, and here is what I've observed

• Aggro is initially obtained through proximity or attacks (even non-damaging attacks)

• Aggro obtained through proximity can be trumped by damage, meaning if you do damage you can pull aggro off of someone who had obtained it through proximity only.

• You do not pull aggro from doing more damage than another person once aggro is obtained via damage

• Mobs will frequently and randomly decide to stop hitting their target, and look for someone else to hit. Once this happens, aggro can be re-obtained through the same thing: either proximity or attacks

• Skills which interrupt mobs, including those which push/pull mobs around, will cause monsters to find a new target after they've recovered

• Boss fights seem to work the same as regular mob fights

Some things I have noticed to back this theory up:

• If I aggro monsters by being too close to them and my DPS hits them before I do, the monsters run for the DPS player

• If I pull monsters with a damaging ability, such as Cyclone Strike, I keep agro for a few seconds regardless of how much damage the dps players do until monsters randomly start dropping aggro and looking for a new target

• Tanking monsters with an AOE dps ability active, such as Sweeping Wind, typically will keep monsters on me since when they drop agro, my aoe ability will tick and cause them to stay on me. Of course, it's not perfect since dps has a chance to hit before my aoe ability ticks.

• Using a skill like Cyclone Strike to pull a mob off a dps player will cause the monster to re-focus on me in most cases, unless they take damage from the dps player before I can do damage, or are shielded/invulnerable so I don't do damage

Other notes...

• Per Bashiok's twitter comment, some mobs may work differently

• Damage absorbed by the enemy does not count for aggro purposes (for example, hitting a shielded enemy will not pull aggro off of someone who has it via proximity only)

• Much of this is simply speculation based on what I've observed while playing, and the little bit of information found online.

As for controlling aggro....

Here's what I find works best for when you want aggro:

• Pull mobs by doing damage, so dps doesn't pull them off you with their damage

• Use AOE abilities if possible so monsters will re-target you after losing interest

• If you don't use AOE abilities, consider building attack speed so you can pick up monsters faster when they lose interest in you

• Have some damage reflection, so things that aggro to you based on proximity will now aggro because of damage, and won't get pulled off by other dps

• Be wary of monsters losing interest with you, and use push/pull/stun/etc skills to reset their target and hit them immediately afterwards to pick aggro back up

• Hit things chasing squishy people, because their aggro resets fairly frequently and you will probably pick it back up quickly

For not getting aggro:

• Let your tank do damage first

• Stand back, so your tank gets proximity aggro when a mob loses focus, and they have a chance to do damage and pick them back up

• Use push/stun/blind/etc skills if you get aggro to reset the mob's target

• Stop hitting things chasing you if the tank is hitting it as well if you want the tank to pick aggro back up

Most of this stuff is probably just common sense, but figured I'd list it to provide a complete answer to the question.

I decided to test this by removing my gear and all increased attack speed. I then test the weapon I chose for my main hand, over 400 base damage with 1.60 attack speed. During Rapid Fire I was averaging 800 never below 450. I then tested weapon #2 which i later put in offhand barley over 200 damage and used rapid fire. I was averaging around 200, never over 375. I then duel wielded each weapon. #1 in main and # 2 in offhand. I recorded myself shooting creatures with short bursts of rapid fire. The outcome was this.

Barrage #1: 253, 133, 58, 301

Barrage #2: 898, 882, 884, 715

Barrage #3: 636, 100, 469 (crit), 156 (crit)

Barrage #4: 532, 649, 544 (crit), 490

Barrage #5: 80, 185, 84, 122

Barrage #6: 220 (crit), 1200 (crit), 823, 743

In barrage one you can see that all 4 shots were fired from the offhand due to none of them deals over 400 damage. In barrage #2 you can see all 4 shots are from the main hand, due to all the weapons damage being over 500, but in barrage #3 you can see both the main hand and the off hand dealing damage. the main hand being the 636 and the off hand being the rest of the damage in the barrage. In barrage #4 I believe that the main hand did all of the damage due to the fact the the off hand never crit for over 375. This also makes Barrage #4 mimic #1 but for the opposite hand. In #5 it is all off hand damage that mimics #2 except for the opposite hand and in #6 it is mixed again with the offhand dealing the 220 crit and the main hand dealing the rest, but still mimics #3.

Answer:

As you can see each weapon has its own consistent damage. It also appears that each weapon has some sort of a cast time and a rhythm that barrage 1 and 4, 2 and 5, and 5 and 6 mimic each other, but is even. The only differences is which weapon was dominate in the set of attacks. If each alternate weapon did damage for each alternate hit the damage would look more like 800,200,800,200. I suspect that weapons with different attack speed would have a much different outcome, and the dominate weapon being the one with the most attack speed.