I like the "take -10 to hit for an auto-crit" idea better. I don't know that I'd say the game is balanced around players only hitting 50% of the time - many of the pure Strikers in my game are regularly attaining odds a lot better than that. My barbarian hits your typical brute/minion/artillery on something like a 7+, generally.
Keep in mind, though, that critical hits often trigger other features. Take the barbarian as an example: in addition to possibly wielding a weapon that does additional damage on a crit (both via magical item property or simply being a high-crit weapon), he gets to follow that up with another melee basic attack. Assuming he kills his target (which to be frank is pretty damn likely at that point), he can potentially follow that up with a charge.
As your players get higher level, the number of class- and feat- based mechanics that are triggered on critical hits only grows, and abilities like Righteous Brand/Lance of Faith can set up situations that offset that Called Shot to a great degree. This isn't to say it's a BAD idea (players smart enough to stack bonuses up to a point where Called Shots have some likelihood to hit should be rewarded!), but really think through the size of the penalty and make sure it's offsetting the potentially massive benefits they may reap for getting a critical hit. Given how easy it is to crank your attack bonus up really high for a single attack and/or get attack rerolls/multiple rolls/adjust roles you don't like (e.g. Oath of Enmity, the barbarian power Combat Surge, the Deva and Elf racial powers, the human feat Action Surge, etc), this seems like it could have some consequences you don't intend. It's not hard to envision a situation in which a striker spends an AP, gets a solid leader bonus power, uses a multiple roll/reroll a miss ability, etc. until he's practically guaranteed that crit, possibly on a 3W or 4W damage power. Then he scores a ton of extra damage, your cleric pops a channel divinity power that triggers on a crit, etc. I think there's a reason why even at epic tier a lot of strikers are still only critting at on a 19 or 20!
Finally, it's also worth noting that like most abilities that reward getting lucky on dice rolls, it can seriously wreak havoc on your players if monsters get access to it, too. That solo who's designed to hit your fighter ~70% of the time will have absolutely no trouble making called shots against the wizard 60% of the time, if not more often. An auto-crit or two could literally knock him unconscious and/or accidentally kill him before anybody has a chance to help him out. If your players are doing that to monsters, you can always add an extra monster to future encounters...if that happens to the party's wizard, they're buying a resurrection or he's rolling up a new character.
Anyway, don't mean to take the wind out of your sails, just pointing out some concerns. It's a really interesting idea and now that you've mentioned it, I'm considering it as an environmental/terrain power for some upcoming encounters in my campaign (my players are only level 2, and in early heroic tier I'm a big fan of giving players options besides "ok, encounter powers are gone, everyone spam at-wills at the monsters until they're dead"). I just think it has a lot of implications - particularly as Strikers get more and more abilities that ensure their hits will land - to think through before I'd roll it out as a general house rule for my campaign.