A game that does not acknowledge the old chess maxim, "The threat is stronger than the execution," has a problem. One of the problems this game has is the need to create specific mechanics for creatures to attempt to do something that should be plausible, if not always likely, for anything to try.
Or knocked unconscious, as is always the option. That lends itself to the possibility of a second tier of options: you can either grab and do improvised weapon damage or you can treat the target as restrained. A second standard action against a target thus restrained could then either join the grab and do improvised weapon damage or render the target helpless until the grab is escaped. Too good? That depends on what else changes: minion mechanics, adjudication of attack rolls, options for investing your "luck" toward another's survival chances, etc.
As you note, the readied action to finish off a helpless target is already a pretty hefty investment of resources, especially when the target is still conscious and there's a defender to consider. I don't allow readied actions to interrupt triggering events that weren't detected until after they happened, and might even adjust what the response is or even allow none at all if the creature with the readied action would have no way of knowing the trigger happened even after it finished.
If people have to maneuver around for a round or two before an attack power can be used, I'm perfectly fine with that. Players should not be conditioned to reach for those things as their first option, nor to think that you should always use all of your encounter powers in a given fight (save perhaps those that always trigger off a successful basic attack). The results are like a chess game in which a player only moves two or three pieces all game.
I haven't made myself clear. Basic attacks or at-wills should not be preferable options. Likewise, encounter attack powers should not be preferable to at-will powers or basic attacks. These things limit choice, which in turn limits thinking.
For example, the mechanics for aiding another in combat could also do with an overhaul. A big part of that involves getting rid of powers or features that force enemies to target you instead or even grant allies a basic attack. By forcing players to invest resources in such options, you ensure that they are never options for people who didn't build along such lines, limiting what they will try. Limiting what your players will attempt is bad, and is one of the things people most criticize about this game.
It should be enough that your character is strong (and has a hand or two free) to consider grabbing your enemy a viable option. It should be enough that your character is quick to consider throwing off your enemy's aim against a given target, to an extent that gives you nothing if the enemy says, "Okay, then I'll just attack you instead," while still involving some risk to either you or the intended target (or maybe both). An aided attack should offer something more than a bonus to hit for what you're giving up, something it does not do when even your at-will or basic melee attacks offer more than someone else gets from a +2 bonus on the attack roll.
For example, if enemy hits the target whose defense you were aiding, make a saving throw. Success means the enemy must target you instead. Failure means the original target is still attacked, with the normal bonus to the target's defenses. A roll of 1 on the saving throw means the target gets to attack you both. If your defenses are being aided by multiple allies (up to four in most cases), the target enemy gets to choose which of the successful defenders to hit, but gets to attack any of them that rolled a 1 on the saving throw as a free action.
Another example: With an aided attack, the target makes a saving throw. If it succeeds, the aid merely adds the normal bonus to the attack roll. If it fails, you get to make a melee basic attack as an opportunity action against the trigger if your aided ally hits the target, using the same bonus you provided.
Fewer powers. More teamwork.
The rules already cover that by making it a Strength based attack, and not even a melee basic attack.