Soooo, I finally read the original Watchmen. It's really great for the most part, with a few hiccups in the plot here and there. I do, however, prefer the movie's ending because I feel it makes more sense within the world presented, and the conclusion set up better in many ways. I don't, however, want to get into an argument concerning that issue. I just thought it interesting to bring up the thing about Dr Manhattan being brought into the main DCU.
Lots of people have said that Watchmen as a work in its own right is perfect as is, that it's best left alone and any addition or expansion is pointless. One only need look at the reaction fans had to the "Before Watchmen" prequel comics to see, people don't really want to see that world revisited, that it's needless. It's also been said that making Dr Manhattan responsible for the DC Universe changing as it has is forced, needless etc.
However, having now finished reading the original 1986 comic, I can say... I think it makes a lot of sense what DC's doing right now. See, at the end of Watchmen (minor spoilers) Manhattan decides he's had enough with this version of humanity, that life on this world is too complicated and difficult to comprehend. He says he wants to leave for different galaxies. Maybe make some worlds and life himself. He's clearly interested in experimenting with the universe and life itself:
Hell, looking at the page, he specifically says not only has he "regained interest in human life," he says he wants to make some. MAKE some human life. And with that, he'll likely experiment with it. With the power that he wields, it's likely he can also traverse universes if he wanted to, so him winding up in the DC Universe isn't that much of a stretch. Also bear in mind, despite what complexities have been introduced in the DCU over the years, things are ultimately a lot more simple there than in either the real world or Watchmen. For the most part, villains are bad guys, and heroes are good guys. Yes, there's wiggle room, grey areas and different aspects to various characters, but it's generally true that DC has polarised characters.
To Manhattan, I can imagine DC's world would present something fascinating; a complex world, with nonetheless relatively simplistic morals for the most part.
All of that is basically my round-about way of saying, after many months of thinking it through, I'm totally okay with Dr Manhattan in the DCU. I get why people don't like the idea, but it doesn't bother me. And I'm definitely looking forward to The Doomsday Clock.