I recently posted a “rites” mechanic, where a wizard spends a turn powering-up his spells. I think it might actually be a good general mechanic: every class has a unique way to spend a turn powering up an attack; if successful, all hits become critical hits.
Rogue: Skulk. As a turn-ending standard action, a rogue makes a Hide check vs. an opponent. If successful, the first hit on that opponent next turn will be a critical hit.
Fighter: Windup. A fighter begins a combat move. Like the wizard, if the fighter is hit before the next turn, the move is spoiled.
Cleric: Abjure. As a standard action, the cleric flourishes a holy symbol and demands that all enemies retreat. The next turn, an attack on any enemy within 3 squares of the cleric is a critical hit.