Welcome to the believability problem many of us have with AEDU, particularly the D part for martial exploits. If the PC can force the opportunity once, why can't he force it again? We need a more believable model than the PC can only do so once a day. Fatigue would be a more useful model than the player's narrative control.
I don't particularly have a believability problem with AEDU; it's a way to provide occasional openings for martial exploits - it makes sense to me on that basis. Could you use "fatigue" or something instead? Sure, but I think it would add complexity for (to me) little gain. For me, you lose far more removing those opportunities altogether than you do by abstracting the timing to player choice.
You are looking for a difference that isn't there. Critical hits in 3e and 4e play the exact same role. What's different is 3e tried to provide different behavior profiles for different weapons - somewhat more common vs less common but doing more damage (both of which even out in the long run) - so that the different weapons chosen actually mean something in play. That's pretty much it.
Well, the difference
is there because 3.x "x2" damage is a probability distribtuion with variability that goes well above the maximum "non-critical" hit level, but am I seeing designer intention where there was none? Quite possibly. That still leaves 4e criticals fitting the "perfect shot" niche quite effectively for my own purposes, though, thanks very much.
Oh, and 4e differentiates weapons pretty effectively without the crit multipliers, so although I can see the utility of that in 3.x, it's not really needed in 4e.
If it's at the fighter PC's will to use the power, that's the fighter causing the opportunity to happen. That's what I mean by forcing the opportunity - taking as he desires and at his will. If the opportunities are serendipitous, isn't that more like the luck factor of achieving a critical hit?
That still leaves out arguably the most common case, though; where the possible opportunity becomes clear, but the fighter needs to make some sort of deliberate gambit to open up a weakness as a result. If I interpret from an opponent's body language and sensory focus what his/her immediate goal is, that doesn't give me an opening - but it
does give me a tool with which to
plan a deception that
may well lead to an opening...
Situation, serendipity and skill are all required, but it's a bit like the "fire triangle". Folk say you need three things for a fire - fuel, oxygen and a source of ignition. But fuel is found all over the place and oxygen is literally almost everywhere, so if you have a source of ignition (a skilled fighter, in this case) you'll almost assuredly have a fire. Likewise, put a
skilled fighter into a situation and they will almost certainly find an opening. They probably won't be able to tell you, in advance, what the opening will be - but they'll find one.