For example, Superman is literally the most powerful man on Earth and is a top contender universe wide, but the problem is that I have very rarely seen him make mistakes or come off his moral high horse. The only times he does seem to be the times that are most remembered, such as his killing of Doomsday (which cost him his life), the Justice Lords timeline, or the time he went ballistic on Mongul for the Black Mercy Plant incident. Other than that, he's always been extremely virtuous and boy-scout like.
DC comic character were always stalwart and true (reflecting the era they came from), paragons of the American ideal. Even Marvel's Captain America was the same way.
Of course, a lot of the steam was taken out of DC due to the Comics Code Authority of the 1950s, which took a lot of what made their comics popular and dumbed it down.
Marvel characters, which came around in the 1960's, are very much human. They have real world problems, such as prejudices, unemployment, unrequited affections, disabilities, etc. They make mistakes. COSTLY mistakes. Even Captain America, despite having come from a different era (or perhaps because of it), made mistakes.
This vulnerability with Marvel characters (which appealed to the American public), coupled with the CCA's intervention on DC Comics, caused some disillusionment with DC and Marvel took off. They still loved their DC heroes, but for different reasons. This is why, overall, Marvel characters have had more movie contracts than DC. The characters are more human, and because of that, the stories can be translated better to the big screen. The only issue with the movies have arisen because of bad casting (Jessica Alba, Eric Bana, Halle Berry), poor writing (Daredevil, Elektra, Ang Lee's Hulk), or pandering to the lowest common denominator (Jessica Alba again). All of that can be pretty much attributed to Executive Meddling. The studios had them by the cajones and made them do what they wanted. Once Marvel Studios came about, the movies have been pretty spot on excellent.
DC Movies over the years have had a good track record, though. Superman the Movie was awesome and still is today, as is Superman II. Superman III and IV were almost iconic depictions of stupidity in production. Then came Superman Returns, a spirited sequel that ignored III and IV. There was a lot of potential in that movie (I thought Brandon Routh channeled Christopher Reeve as both Clark Kent and Superman quite well), but the overall inherent stupidity (Really, Lex? Another freaking land scam?) and some poor casting (Kate Bosworth looked more like a college student than a famous reporter... that and I wanted to feed her a truckload of sandwiches) made the movie fall kinda flat.
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