[Crazy Jetty]

To me... it's how well the creators hold onto the spirit of the character they're adapting. It's not nessisarily about how accurate it is.
Arrow and the Dark Knight Trilogy are great examples of this.
Both have a lot of changes to them that make them less accurate to their comic counterparts, but both are cases to make it a better fit for adaptation, to make them work better in the more real-world settings a live action project brings... but both keep to the heart of the character.
Watching Arrow, I never once get the feeling "This is okay... but that's not Ollie," like I did with Hal Jordan in the Green Lantern movie, or Peter Parker in Raimi's Spiderman Trilogy.
And it's not because Oliver Queen is represented more accurately in Arrow, than those others. He's really not, especially since he kills pretty easily.. But its that they really seem to understand the character and the concept better. And the changes made are in order to make the adaptation work better.

What I can't stand, are changes that are made just to make changes: The previous four Batman films, Burton's failed take on Superman, these are pretty good examples.