It's also worth noting that monsters tend to have more variation in their damage then pcs. A paragon level pc might do d10+13, while a paragon level monster is more likely to do 2d10+7. This leads to the same average damage, but causes the monsters crits to be more deadly before any extra damage dice are added. Strikers are of course an exception to this with all their extra dice (though phb2 strikers appear to have flat damage bonuses or other mechanics, weakening their crits).
Yeah, I think that's something that's missing out of this. Part of the difference between PC's and Monsters is that a lot of monsters already roll a pile of dice to begin with...and then that pile gets maxed. Take an Ancient Red Dragon, for example. It's a Level 30 Solo Soldier whose At-Will Claw attack does 2d12+12 + 6d6 Fire Damage. On average this comes out to 13+12 + 21 Fire Damage, for a total of 46 damage. On a crit, that pile becomes maximized and we get 24+12 + 36 Fire Damage for a total of 72.
Now, PC At-Will powers don't make that big of a jump because they're not rolling that many dice...well, except for the Rogue. But this is a Soldier Solo, so it's best to compare it to the Fighter. At Best, a normal Fighter will be rolling 4d6 (High Crit) with an Executioner's Axe. Looking at what I would consider to be a more "average" scenario and we'll probably be seeing 2d10 or 2d12 without the High Crit property. Possibly even as low as 2d8 if they were using a Spear or Longsword.
That's just an At-Will comparison though... Granted, a 30th level Fighter could pull out a 7[W] Daily and roll a crit on it, plus his magic weapon dice, but as it's a daily that's a bit more rare. Looking at the Encounter powers you're not getting near that high. You might be getting like 4[W] with something like Skullcrusher though. Compare that to a crit with the Dragon's Immolate Foe power though. The PC might edge out in damage due to the extra magic weapon dice, but it won't recharge in about 3 rounds (it recharges on a 5 or 6, which is about a 1 in 3 chance).
The other thing to remember here is that NPC's don't play like PC's. There's plenty of NPC's, especially amongst the Solos and Elites, that get multiple attacks per round At-Will. Then they might also have a couple of action points, but they can use more than per encounter. So, with these BBEG type monsters they're putting out a lot more attacks that a similar PC has access to, which gives them a higher chance to crit.
While the non-Solo/Elite monsters might not have as many attacks, they will typically outnumber the party, thus giving the DM more d20 rolls than the party and more chances for a crit. By tossing extra dice on top of this it could result in a bloodbath for the PC's if the DM happens to roll a couple 20's within a round or two of each other.
In our game we're curretly going through KotS, so we're low level and extra crit dice wouldn't come into play anyway...but just as an example of how things can quickly get out of hand you can look at our last session. I (The Fighter) was holding off a group of 3 Soldiers in a side room while my allies were dealing with the rest of the party. They had their hands full with at least 1 target per PC, which is why I was trying to hold the reinforcements off. I got hit by crits
3 rounds in a row. The only reason I was even able to survive was because we have two Clerics which means that they could afford to spend 2 Healing Words on me (That and I'm a high CON Dragonborn with Toughness...). This eneded up happening because of the superior numbers the DM had. While statistically unlikely, he was rolling 3 attacks to my 1 attack each round. That means that his chances to crit are 3 times as high as mine...which also means that if each crit was as strong as a PC's then it would be VERY hard to survive mutiple crits, which is essentially an inevitability due to the NPC to PC ratio.