Q: Watchmen ate three hours of my life. Why?
A: Director Zack Snyder desperately wanted to be faithful to the source material, both out of respect for the Watchmen graphic novel and to prevent his death at the hands of Watchmen fanatics. Twelve issues of comics + fastidious translation = three-hour movie.
Q: I thought this was going to be a superhero movie. What gives?
A: Alan Moore wrote Watchmen as a deconstruction of the genre disguised as a mid-‘80s superhero murder mystery in an attempt to get mainstream respectability for the comics medium. Those scenes with all the words and no fighting were in the originals, too.
Q: Is that why the blue guy keeps forgetting his pants?
A: Kind of. Watchmen's heroes are the answer to the question, "What kind of nutjob would actually dress up in a costume and punch people?" Dr. Manhattan loses touch with his humanity, The Comedian is a sadist, Rorschach has issues with subjective morality, etc. Watchmen jumps back and forth between decades; if Dr. Manhattan isn't wearing underpants and disintegrating the Vietcong, it's probably the 1980s.
Q: The 1980s? President Nixon resigned in 1974!
A: Alternate realities sure are wacky! In Watchmen history, Watergate was never exposed, and Nixon was subsequently elected to five terms. This sets a decidedly more antagonistic geopolitical stage.
Q: So where did everyone else get their powers?
A: Dr. Manhattan is the only character in Watchmen with actual superpowers. Everyone else is left to fend for themselves with a combination of fighting prowess and whatever psychoses push them toward the crime-fighting lifestyle. If the fight scenes look unrealistic, blame that on Snyder's tenuous grasp of physics. (See also: 300.)
Q: Why are there two Nite Owls?
A: Watchmen spans several decades, and consequently several superhero teams. Some of the latter-day heroes took up the mantle of their predecessors, either out of respect (Nite Owl II) or to keep the family business alive (Silk Spectre II). Dr. Manhattan is functionally immortal, and others either came along after the "first wave" or just got old.