He's certainly upping the lethality, but it's not wrong nor broken, AFAICT.
If doubling seems too much, but nothing seems too little, consider flat bonus damage based on tier: +5 damage if a minion scores a crit in heroic tier, +10 damage in paragon tier, and +15 damage in epic tier. (Or alternatively, +10, +20, and +30 for grittier combats.)
Do bear in mind, that minions don't do their damage alone, but as the result of the chipping away of the entire group effort.
Critical hits' addition to damage from minions is actually a lot more severe than from a normal, because the minion can do this critical hit a lot more often.
Think of it this way... let's say joe blow minion has 50% chance to hit you, with an even 4 damage per hit. Four of them together will do damage under normal circumstances on the following curve:
1/16th of the time: 0 damage
4/16th of the time: 4 damage
6/16th of the time: 8 damage
4/16th of the time: 12 damage
1/16th of the time: 16 damage
However, with crit for double, that means that 1/10th of all hits actually do double damage, which changes the curve signifigantly. With four minions, that damage which -should- happen every 20 rounds or so is now happening every 5 rounds with regularity. It makes a much larger difference with minions than it does with other monsters that attack less often.
Considering that damage spikes in minions are -already- inherent in the fact they get more attacks... AND that four minions' damage is actually more on average than a singular monster's damage, and more reliable and steady due to law of averages kicking in more and more often.... adding critical hits doubling damage to them might not be such a good idea. It's one of those rules changes that can only punish players, as players -never- get to take advantage of it, so to them it just becomes a way for you to 'do moar damage.
You can accompish -that- goal easier by simply increasing minion damage by a point.