My theory is a simple one. The more faithful the adaptation is to the source material, the more successful the film or TV show will be.
The provisions on this theory though are as follows:
1. The comicbook needs to be popular- otherwise there won't be an inbuilt audience and all bets are off, effectively the movie or show is a blank slate to succeed or fail on its own. Example: BLADE movies.
2. The adaptation needs to stay true to the source material's spirit. The 1960s Batman television show was actually LESS "zany" than the contemporary comicbook but certainly catered to the same audience. Likewise today's Batman adaptations have to be all grim and gritty to gloss over the inherent silliness of the character.
I really think that the single biggest flaw in adaptations is when some more or less talentless hack cops out and tries to "boldly reimagine" something that is wonderful and successful and doesn't need the help of some intellectual dwarf to be a big hit. Harry Potter for example just needed to get to the big screen unmolested so that its legion of fans could all troop off and have their prejudices confirmed. Wholesale changes to a story or its characters is a betrayal of the source material, a bait and switch on the poor fools who give up (far too much) money to go and see a film in the sticky sewers we call cinemas and fundamentally indicates how stupid and badly educated most film makers who do adaptations are.
Note to film makers everywhere- YOU ARE NOT SMARTER than the genius who wrote the original classic book or comicbook. Get over yourselves.