Originally posted by Andrew Dynon
Good Idea: Enhancing the feel of your Champions game by using Danny Elfman's themes for "Batman", "The Flash" and "Spider-Man".
Bad Idea: Enhancing the feel of your Dark Champions game by using Danny Elfman's theme for "The Simpsons".

And not Hero, but...
Good Idea: Charging into melee combat with a bunch of gun-weilding mooks in Feng Shui.
Bad Idea: Charging into melee combat with a bunch of gun-weilding mooks in Shadowrun.

(I've done it in both systems, so I speak from experience)
In a related note:

Good Idea: Increasing the excitment by describing your combat actions in dramatic detail.
Bad Idea: "I draw my twohanded sword, scream at the top of my lungs and charge from the <b>Back</b> of the party, swinging in haphazard, sweeping arcs."

- lest you want to be killed by the return fire of your own party...