I think this is what is one of the best aspects of the designers of 4E getting the "math right". Because now... using the "DM's perogative" of adding a +2 or -2 to any target number to do something outside of the rules as written, actually has an appreciable affect and is useful. Especially when it comes to adjucating the amorphous "Acrobatic Stunt" action (which is written into the Acrobatics skill, but has no mechanical rules to apply to anything).
In the games I've DM'd... when players ask about knocking their opponents over tables, leaping off of bars and over their opponents to avoid OppAtts, throwing plates at someone's hand to make them drop a scroll... I just select TNs, apply any DM perogratives as needed, and let them do it (even if these sort of Trip & Disarm attacks and movement ideas aren't actually written into the rules).
It just makes things that much more fun for people. Why stop them from doing these kind of stunts if they come up with them?