It'll be a wretched, disgusting failure. Think about Dark Knight. What made Dark Knight awesome? It was original. I know that sounds nuts, I can hear you saying "Uh, Crunchy, I don't know if you know this, but Batman is like, the most prolific modern character in existence." And that's true, which is why it is so much more impressive that the Dark Knight did its own thing. The Dark Knight's Batman is not Burton's Batman, or Schumacher's Batman or even Kane's Batman. The Dark Knight's Joker is a different character than what we've seen in the past. And The Dark Knight's Two-Face was perhaps the finest iteration I've ever seen, but it owes little if anything to the Animated series, or the comics, or, God Help Us, Batman Forever.
Technically, Batman Begins and its sequels are rooted in "The Long Hallowe'en" but I think it's fair to say that the creators of these films broke any and all barriers in the comic they felt they needed too.
Watchman on the other hand, seems to be trying to placate the ravenous Watchmen fans. The Watchmen story has only been told once, and most people agree it was told as well as it ever could be. Snyder will do his shot-for-shot remake to try stop from upsetting the die hards, and ruin his chances at making a decent movie out of it in the process.
Umm, maybe that was too long a post, I think my point is pretty simple though:
"In order for a movie adaptation to be successful, its creators have to be able to tell the difference between vital themes of a story, and useless minutia. Dark Knight had a team that could do that. I'm not confident Watchmen does. And that's probably just because Watchmen deals with more complicated themes."