One reason why Alan Moore (like lots of other people) may have thought that Watchmen was unfilmable was the use of subtle associations and tiny messages that could only be revealed by long scrutiny of the individual pages and panels. According to Moore, in Watchmen we see:
sort of “under-language” at work ... that is neither the “visuals” nor the “verbals,” but a unique effect caused by a combination of the two. A picture can be set against text ironically, or it can be used to support the text, or it can be completely disjointed from the text - which forces the reader into looking at the scene in a new way.... the reader has the ability to stop and linger over one particular “frame” and work out all of the meaning in that frame or panel. (Quoted in "Reading Space in Watchmen.")
Well, movies don't allow that same kind of attention at full speed in the theater. They DO allow it in the freeze-frame -- and Zack Snyder's Watchmen title sequence actually slows down and freezes the frame for you. Now Meredith Woerner's got the goods on the easter eggs in the title sequence for Watchmen, and at least one is a doozy:
The opening shot, with Nite Owl giving a fist full of justice has a big Batman reference. First, check out the posters to the right. Look familiar? And isn't that Mr. and Mrs. Wayne at the back entrance of the opera, being saved from a bloody death? And according to commenter Rainbucket, the opera bills say: "Die Fledermaus" (The Bat). So can we safely come to the conclusion that the original Nite Owl stopped Batman from popping up in their universe?