For example : When the ranger chose not to use twin strike and use his grapling hook to try to grapple one Guard Drake and bring him down rise he was on and rolled a one, I rolled one d20 and rolled a 3 so I ruled he had caught the Minion behind, asked for a strength roll, he rolled high. I said you bring the minion down killing him and knock prone the drake. Next thing that happened, you saw both of the player try to use the world instead of using powers exclusively.
While I agree with the idea in principle, and minions are good fodder to use for dramatic flourish, I would like to say that there should be some consistency in the approach to ad-hoc maneuvers. Having the maneuver fail without a spectacular fumble roll, yet achieving a better effect than a normal success, is more DM fiat than a consistent standard for ad-hoc maneuvers.
Of course, DM fiat is a fine tool to use on its own when you're not sure of the rules and want to nudge things in favor of dramaticism. As a new DM with possibly new players, it is probably recommended as well. But, as you all become more comfortable with the game, don't be afraid to let the dice fall where they will and take the critical hits and fumbles by the chin.