OK, I'll start by saying that I haven't watch or read any of this clasic, JET!! BUT I know how you feel. I read The Watchmen comic and later I watch the movie (guess I was WATCHING THE WATCHMEN? YEEEAAAHH.... sorry) and also felt that some good points in the history were missed, but THEN lets remember that this is a very, very deep story and in order to keep it at a movie length, sacrifices had to be done; not that it made a bad movie, nooo, I watched that movie 3 times back then and 4 times every time it's shown on the tv.
It's hard to make something good, and even harder to change it to another format. I like to think that it's like the ''wave theory''; in phisics, a certain wave (light, sound, etc) moves faster if the material that its traveling through is less dense(gas > liquid > solid) and if it passes from one state of matter to another(specially if it's more dense) it will lose some speed. So think about this, gas would be literature(books, comics, etc) where it has all the time develop itself, and later on are the adaptations, where solid is movies and liquid would be tv series or even video games.
So, there you go. I hope that this reply was both helpfull and enlightning. Have a good day/night.