I loved this movie. It's more of an epic sci-fi saga than a super-hero movie. In a way it's more like Star Wars if you take out the Jedi and whiny actors while keeping the setting and kicking the "sense of wonder" up a notch. It is set in the Marvel movie-verse but you don't need to see the other movies to enjoy this one. Conversely if you don't see it, about all you need to know is that the stakes for the plot involving Thanos (hinted at in Avengers) just got a lot higher.
One of the many things they got right about this movie is working in bits of the Marvel comic book setting into the movie without making it important. Some things are introduced, told what we need to know about it, then we move on. It's window dressing for the fans. If you recognize something, great, if not that's OK too.
After the movie they have two bits before and after the credits (not during). They're both funny but neither foreshadow anything in the other films. They're not the "I want to tell you about the Avengers Initiative" kind of thing that made our jaw drop at the end of Iron Man. So if you drank the Bladder-Buster sized drink at the start of the movie and need to visit the privy, go ahead after the first one. You may make it back in time for the second, but if not you'd just miss an inside joke.
I know there's a cartoon in the works, but I'm not sure about a sequel. I don't know enough about GoG lore to say if there's something that would work in a sequel.
My only concern about the movie is that since it is more sci-fi than super-hero, some people may not be interested. To those people I say, "You're making a huge mistake. Go see it. It may not have super-heroes but it does have super-hero style action."