I'm keeping an open minded on this one. On the one hand - I really respect Zack Synder, I think he really is "visionary" as quoted in the trailer. His cinematography in Dawn of the Dead and 300 were excellent, in terms of visual, timing and flow etc.
300 might have been a "mistake" in that the comic wasn't really suited for a film - the non-linear narrative and structure of the story was perfect as a comic or a short tv-series.
However, I think Zack respected the differences in the mediums, and instead of doing what Robert Rodriguez did with Sin City (copying things panel by panel, which I think further cemented him in my mind as a weak and mere "cult" director, and I thought the film flowed really poorly and just felt tacky), Zack actually rewrote 300 as a film, even if it meant changing the essence of the story.
In that I think it was more "inspired" by 300 than an "adaptation" of 300.
With Watchmen however, Zack's actually adapting the graphics novel. I trust that he has an understanding of the differences in the medium, and I think he will make it "work" at least as a "film". Problem is of course, it would lose many aspect of the graphics novel, but I think Zack would actually create something "new" for the film by taking advantage of the motion picture medium.
Undoubtfully many fanboys of Watchmen would slate it regardless for not being true to the original. As for me, I'm more interested in whether it'll actually be a good film, rather than religiously comparing it to the novel. However I do expect Zack Synder to maintain the essence of the Watchmen's story at least.
The change of the ending does confuse me hugely as well. I don't think it's a "slight" change at all considering the whole "point" of the plot... We'll have to see!