I am... uncertain about the balancing through recharge abilities. I am looking forward to seeing what they'll put forth, but I feel it will change the balance.
Mechanically, monster crits are a 5% change doing doing double damage, each attack roll. In my experience, fights hardly last more than 3-4 rounds, so that's, say 8 attacks per monster, assuming they have two attacks each. Let's say, roughly, that you'd expect a critical each fight when you face three of these monsers (I know it's not how the maths work and you'd get roughly 1 in 3 chance of having a fight without crit), but the please bear with me, the order of magnitude is the important thing here).
Recharge happen on a 6 or on a 5-6 each round. Also, monsters start with the power charged. So, against a fight of 3 monsters for 4 rounds, you'll expect 3 recharge power (the initial use) and, over 12 rolls, 2 or 4 recharged use (insert math disclaimer). That makes recharge power happening 5-7 times over the course of a single fight. That means the recharge power isn't a huge thing, since it must be balanced with "double damage" happening less often. It would be:
Regular attack: 2d8 damage
Mighty blow (recharge 5-6): 2d10 damage.
Not as exciting as a a crit IMO, high risk, low chance (and in fact I wouldn't bother with the recharge rolls if this was the case...), that you need to prepare for. "I fear to kill my players" "OK, then tell them that attacking a vicious-looking murderer with a big, pointy metal stick in his hand is DANGEROUS and you'd better enter the fight with a healing potion." With the death rolls, if you ignore sudden death by massive damage, lethality is already at an all time low and the borderline case where it happens can be prepared for by any careful character.
As a side note, I try to narrate fights but, being totally lazy, there are many times when narrating the 24th sword swing of bugbear #3 is... a very concise narration. However, I'll take time to narrate a crit against a player. It helps make the higher damage memorable, I think.