Senario 1: You are attacking a troll. The troll is unaware of you so you have Combat advantage — +2 to your attack roll.

Senario 2: You are going to attack a troll. Two of your companions have flanked the troll, and the troll is dazed from an attack by your warlock companion. You have remained hidden so the troll is unaware of you. You have combat advantage 2 ways (dazed, unaware), or 3 ways in my reckoning (flanked, dazed, unaware), but your bonus to your attack roll is still +2.

When fighting stronger opponents it is very helpful to be able to stack as many tactical advantages as possible to increase your chances of hitting and doing damage — in other words wolf pack tactics. But it doesn’t make sense to stack some of the conditions (like Surprised and Stunned). My rule:

Combat Advantage (Mental): The defender’s mental condition has been compromised such that it is less able to defend itself. The attacker gains a +2 Mental Combat Advantage bonus to attack if the defender is Dazed, Helpless, Stunned, Surprised, or Unconscious.

Combat Advantage (Physical): The defender’s physical situation limits its ability to defend itself. The attacker gains a +2 Physical Combat Advantage bonus to attack if the defender is Balancing, Climbing, Prone (melee only), Restrained, Running, Squeezing.

Combat Advantage (Tactical): The attacker (or his allies) has the defender in a advantageous tactical situation. The attacker gains a +2 Tactical Combat Advantage to attack if the defender is Flanked, Unable to see the attacker, or Unaware of the attacker.

Stacking: Combat Advantage bonuses do not stack within type, but an attacker can stack Combat Advantages of multiple types. For example, an attacker would have a +6 bonus against an unconscious opponent with a melee attack (Mental: Unconscious, Helpless; Physical: Prone; Tactical: Unaware of, Unable to see attacker).

This is more complicated but strongly rewards parties (and nasty DMs) when they think creatively to place and opponent in the worst possible situation.

Natural 20: If you roll a 20 on the die when making an attack roll, you score a critical hit if your total attack roll is high enough to hit your target’s defense. If your attack roll is too low to score a critical hit, you still hit automatically.

From a realism perspective, this rule is fine — even though the 1st level Fighter rolled a natural 20 to hit the Ancient Dragon (and thus scored a miraculous hit) she shouldn’t do critical damage because it was a fluke that she could even hit the dragon. But from a game perspective this is just an additional rule complication that shouldn’t be necessary. Just about any opponent you face should be able to be hit by a 20 + your attack bonuses (and the reverse). If not the encounter is not balanced properly. Why not just simplify matters and have a 20 always be critical?

Natural 20: If you roll a 20 on the die when making an attack roll, you score a critical hit.

My rule change would be to add a new condition to the list of conditions on page 277 of the PHB:

Flanked
* You grant combat advantage

You would gain the Flanked condition under the same rules on page 285 of the PHB, but once flanked anyone attacking you would have combat advantage.

This rule change will help encourage parties (and nasty DMs) to use the tactical roles better. If two melee beaters maneuver to flank (perhaps with assistance from a leader), not only will they have an easier time hitting, but the fragile ranged specialists (e.g., bow wielding ranger, warlock, or wizard) will be able to take advantage of the opponents compromised position.