Why is Watchmen considered as one of the greatest creations in not only graphic literature, but in literature in general? It is certainly not the best. We readers are our own subjective tastes. I can give you a list of graphic novels I have read that entertained me more than Watchmen. Alan Moore can be easily beaten by another writer in creativity. Dave Gibbons isn't the best comic book artist of all time. 
 So why is this book, and I will go far with my words, is revered by many as one of the greatest, if not greatest of all time? That it is an iconoclastic and sacrilegious for someone to even touch and more so, expand its lore? Sure, it is era-defining. Watchmen is a definitive example of comic book culture maturity. Certainly, it touches on geo-socio-political events worldwide, depicted so effective and so organic that the things happening in this book are very believable, even though the main characters are nowhere to be seen in the real world. Of yeah, Alan Moore has crafted these Watchmen crimebusters as archetypes of nearly all superheroes in existence: a god, a techie vigilante, an anti-hero vigilante, an uber-cool vigilante, a fan-service heroine and a villain-hero (but honestly, you don't have a Deadpool in Watchmen). 
 So again, why? 
 This is my personal take on it: Watchmen is full of multi-thematic elements, told effectively in a self-contained story. This book has a say on almost anything about comic books, in many levels. Think about a theme in comic books, Watchmen would most likely have it. Simple themes would be godlike heroes, villain-hero relationships, salvation, vigilante, love and sex, crimebusting partners, and the list can go on. How about human frailty, the concept of life in the eyes of a god, the power of media, the repercussions of war, man's apathy on the things going on around him, deliverance from global conflict? I'm not saying that Watchmen has everything. In just covers a lot in its twelve issues. We can appreciate it in many levels: As an entertainment material, indeed you will be entertained without thinking too much about it. As a graphic novel, yeah it is great. As a piece worthy of academic discourse, yeah you can do that too. Every detail is important, every object has a story, everything, the Black Freighter, the hentai octopus giant monster, the smiley pin, the placard, even the Hollis Mason trophy is a piece of conversation. 
 If someone tweaks, or adds something in the Watchmen lore, it is perfectly understandable that people would react, with some finding it sacrilegious. Perfectly understandable. I myself abhorred the idea of a Before Watchmen series (at first), disappointed with the ending of the Snyder's Watchmen film (but enjoyed it) and a bit skeptical (and excited at the same time) with Watchmen's incarnation in the mainstream DC universe. Why? Because this book is self-contained. I don't say that it is error-proof, or all loose ends are tied. Watchmen can just stand on its own. It is one big solid quilt where everything is connected. It has its flaws but far too light too bring the whole graphic novel down. 
 Pick something from Watchmen, anything, and you can get a piece of conversation from it. Review a certain aspect of the graphic novel and you will be astonished on how so many things you can say with that just single aspect. Or you can just say "I enjoyed it so much", and it is perfectly okay. Alan Moore's Watchmen is about so many things. It is an essential literary work that everybody should read.