5- Celebrity Versus Actor- The job title here says it all. Actor. Not celebrity, so why must we rewrite characters to fit their celebrity persona. a good example of this is with Joel Schumacher's horrible Batman films, The Riddler is an intellectual villain who's pathological need to leave clues in the form of riddles could make for a very interesting foe. But hey, we got JIM CARREY, Straight off playing the Mask, Let's make Edward Nigma into a manic comic villain who suits the well known comedy stylings of a multi-million dollar comedy talent. Everyone loved Ace Ventura and Dumb and Dumber, and that's what they expect from Jim Carrey, so instead of a dark serious villain we get a rehashed Jim Carrey routine. Same with casting lovable rogue George Clooney for Batman, gone was the gritty tortured soul of a man who turns his loss of parents into a crusade against crime, and in it's place we get George Clooney, not acting but simply being George Clooney in a benippled batsuit with quips like "This is why Superman works alone." Why was Nolan's Batman a success, ACTING!, look at Cillian Murphy now tagged in Hollywood as a perfect man to play a sociopath, but look at him in the movie he did before Batman Begins, it was called Breakfast on Pluto, and Murphy played about the polar opposite of the Scarecrow in every way, and Liam Niessen played his estranged father, yet on the Batman set you would see none of the tenderness, none of the parental bond, they share in Breakfast on Pluto, instead you get very good straight evil performances that have since cemented Murphy in his celebrity role as a sociopath.
4- Read the Goddamn book!- Sometimes, yes changes have to be made and believe it or not fans get that. I never heard anyone pitch a fit about the fact that Iron Man's origin now takes place in the middle east instead of China, no Balking at Bryan Singer's X-Men movies at the omission of core X-men and adding Wolverine,Storm and Rogue as X-men before they would have been X-men chronologically. Still often times Hollywood finds a need to change it up. Take the film Kick Ass, where 1 single line of dialogue changed the focus, Dave in the comic attributes his origin to a uinque mix of depression and isolation, in the movie Optimism and Obliviousness, see why that might change things. then we have to turn hit girl into a far more sexualized character (really she's like 12, she never had the need to wear a catholic school girl outfit in the book, why put it in the film even going as far to have Dave's friend remark about her being "hot") Further more Big Daddy was simply Crazy in the book, the story of him being this great cop, and being betrayed was a BS story he told his daughter, he was an accountant he chose to attack Demico, because he needed a villain. Instead Hollywood figures audiences can believe the crazy BS story of him being king of cops, over he's just plain nuts. These changes weren't there because we needed to cut time, (hell quite a bit was added to the film as well). So this isn't the X-men trying to cram 40+years of comics into 2 hours or Iron Man's Communist China simply isn't the big threat it once was. Nope this is just change to make a "better" movie. or because they believe the average movie goer isn't smart enough to get concepts like "he's crazy"
3- Pointless Name Checking- This one is a stick wicket, when done right it can be effective to imply things from the greater universe of the comic, such as subtly giving an extra a well known minor character's name. Case in point professor Xavier saying "goodbye Kitty" as a young girl scoops up her books and phases through the wall, we get it, SHE'S KITTY PRIDE!!! but as many times this effect can add insult to injury Green Lantern turned space cop Hal Jordan into a caricature of Ryan Reynold's most popular roles and ignored the point of the character (effectively falling into the two traps we just talked about) but we DO get a treat of using Hector Hammond as the villain, as well as seeing some of the extended Jordan family including his brother Jim. What this says is "see we can make obtuse references to the source, thus we MUST know what we are doing." In reality the ability to cut and paste from a wikipedia article does not make one an expert and often times these names can be used improperly Hammond was nothing like the character in the film (other than the oversized head, kind of) but using this name as the villain sure seems like we know what we are talking about more than say using Sinestro, as he his Jordan's most known foe. In short worry about the big things before you try to BS fans with the little things.
2- Retrograde Modification- Of course even if you do screw something up why not fix it on the back end. This is a relatively new thing but just as annoying. If fans balk at a change simply put it in the book, then it's cannon right? No then you've just screwed up the book on top of screwing up the movie. Judge Dredd was guilty of this when DC got the US rights and issued a new comic that Mirrored the Stalone film, result? No one liked the US Judge Dredd Comic either! and We see this a lot in Green Lantern the "New 52" initiative at DC turned Hal into a warmed over Ryan Reynolds clone, and took Amanda Waller, a woman who was the female equivalent of Nick Fury for DC who had all the sexual appeal of Nell Carter and since she was played by Angela Basset in the film turned her into a smoking hot Female Nick Fury complete with jumping into the field. My reaction? I quit all DC books after the new 52, shame Green Lantern was one of my all time faves.
1- Assuming your audience- I once had a film professor tell me that fanboys are just so overcome with joy that a movie is being made about their favorite hero that they won't care if it's wrong. This is flat out wrong. In fact it is the single dumbest thing I have ever heard about fandom. Fans if anything will be very critical, This doesn't mean we won't forgive some things. Everyone can get basic concepts like Venom's origin cannot be told accurately in the course of a movie as we would have to explain secret wars and Spidey's new suit all of which makes the continuity work to put the Avengers on film look like child's play. But tricks and making things for "joe sixpack" instead of for the fans only assumes we must write the lowest common denominator, and that the comic wasn't a fan favorite for anything but a few esthetics that we can easily clone. Fans are smarter than that, hell non fans are smarter than that. If the medium worked the first step is to look at what made it work and treat those parts like sacred cows, don't change huge swaths of it because you think you can sell more toys or excite non fans with a cameo, if it worked then fans will flock because it was right, and non fans will get turned on to it because it was popular for a reason!