4th edition has gone a long way to eliminating various forms of bookeeping. Admittedly it has introduced new ones in terms of tracking powers and status effects, but mostly I'm finding the combat much smoother. That is except for immediate actions - in particular, limiting them to one per round. At low level it's fine as you'll likely only have one power that will require using one. Later however, when defenders can mark more than one target and when wizards and rangers have two or three reaction abilities, it becomes a real pain having to track whether you've used your action or not.
As for bookkeeping.... it's -really- easy to do so if you use cards. Have four cards, one representing a Standard, Move, and Minor action apiece, and one representing your Immediate. Beginning of your turn, 'untap' them. Tap them when you use them. It works if you have trouble remembering.
You also seemingly can't use them on your turn - leading to some outrageous workarounds of your abilities. I ask whether it's necessary to limit immediate actions to one per round (without putting any research into powers that might abuse multiple immediate actions)
Well, limiting them to one per round means that their use is no longer automatic and assumed, but actually a part of strategy. You have to make a decision whether you want to use your Combat Challenge, or that defensive buff that you're saving up. There's a tension there, and being able to make decisions is more fun than simply responding automaticly to what comes your way (which is what OAs are for.)
and further, why you can't make OAs and immediate actions on your turn. Discuss!
In order for your Immediate or Opportunity actions to be triggered, someone else must be doing something on your turn. If they are doing something on your turn, that means that they are performing an Immediate or Opportunity action on your turn. If this is the case, either someone -else- is also performing an Immediate or Opportunity action, or you're in the middle of performing an action. Even if they are responding to someone else, eventually the chain of actions comes back to you doing something.
So, therefore, if you'd have an opportunity to perform an immediate or opportunity action, you'd already have to be busy doing something else anyways, and therefore unable to perform that other action. Therefore you couldn't perform that immediate or opportunity action, and therefore there's no point permitting you to perform one.