(instead of as in your example, dictating what the stunt does mechanically and again why would I want the DM to decide what I'm doing when I have powers that I know how they work mechanically?)...
Because sometimes you might might want want to do something that falls outside the scope of the guaranteed actions your power grant you.
then the only way to even this out is too make it harder, unless you flat out say no to things that are to powerful... which leads us back to the question of... how do we judge these things?
Wait... There's lots of other ways to make allowances for the power of an effect: action type (standard, move, minor or free action), duration (end of target's next turn, end of attacker's next turn, save ends), area of the effect (single target, burst or blast), repeatablility (at-will, encounter, daily, once only), magnitude of effect (pushing 1 square or pushing 5 squares

), and so on.
We're not making new powers here. Nor should we be. We're using loose guidelines, along with our general knowledge of existing powers and actions, and our previous experiences as a DM to make an impromptu ruling for an action that probably won't happen ever again under the same circumstances.
Edit: As an example, should it be just as easy to push someone 1 square as it is to push them 5 squares?
Oddly enough, when I judge on stunts, Bull Rush and Charging are two of my baselines for what a character can generally do.
All that said, let go back to something else you mention...
...unless you flat out say no to things that are to powerful...
I'm not sure how other DMs run stunts, but here's how it generally goes with me and my players:
The player lays what they want their character to do. Usually, this is a fairly non-rules abused description. To use an example this weekend, the party barbarian got himself stunned while fighting some ghouls. He's still in the front line getting chewed on pretty badly, so my wife, whose rogue is standing right behind him says, "I want to grab him by the collar and drag him back away from the ghouls."
With no real rules for this, but several powers as inspiration, I give her a few suggestions, "You can make an Easy Athletics check as a Move action to pull him back into your square and knock him prone, or you can make a Moderate Athletics check as a Minor action to do the same thing. You can also make a Difficult Athletics check to move half your speed while pulling him with you... If you burn a healing surge, you can turn that into a Moderate Athletics check, or move your full speed."
Once I've laid out the options, she gets to decide which option to take, or she can change her mind and do something else completely different.