Didja' notice the annoying fanboys are all complaining about Marvel stuff, and not DC stuff? I think this is a litmus of the difference between people who are fundamentally Marvel fans (Basement-dwellers) and people who are fundamentally DC fans (People with jobs). The former is more likely to get all worked up over the dauntingly complex canon that is the Marvel universe, whereas the DC Universe has been rebooted so many times that basically you just focus on the characters you like, and ignore the fact that Superman should be like 126 now, or that Captain Atom fought in Vietnam except all of a sudden one day he didn't.
Leaving that highly insulting off-the-cuff observation aside, when asked why "Batman" in the 80s differed so much from the source material, Tim Burton said (Eventually) 'I wasn't copying someone else's story, I was telling my own story.' I think this is fair. If I were doing a superhero movie, I most certainly would *not* use an existing one, though I would use existing characters. Why? Because a movie is not a comic book. It has different sensibilities, and there's no point making a movie if you're just gonna' use the source material as a storyboard. Why bother if you're not going to bring something new to it?
Which was ultimately my problem with Watchmen: It *did* use the source material as a storyboard (Excepting the lame ending), and as a result it was stately, ponderous, boring, and had no life of it's own. It was a stale regurgitation of a tale we all already knew way too well, and frankly it was a waste of time to film it.