Green Lantern was a weird movie for several reasons. I don't think the director, Martin Campbell, understood the tone that was needed for that kind of movie. I think he understands James Bond, but Green Lantern is another matter.
Tonally, the movie was all over the place. Plus, you had Ryan Reynolds, who despite his best intentions was miscast. The movie tried to be at times a serious comic-book movie, a space opera, with some light-hearted humor, and it just didn't work. Campbell and his writers needed to find a tone and stick with it. It didn't help that it tried to be serious, but then had really dumb scenes where Hal Jordan tries to activate the suit and show off to his friend.
The writing could have been a lot better, too. It had some good moments (the Hal Jordan/Carol Ferris dynamic was good, and Reynolds does try with the dramatic stuff) but the handling of the villains was really cliche and over-the-top. It doesn't help that Peter Sarsgaard delivers a completely over-the-top performance, and I think that's hugely as a result of the tone that's completely all over the place. When the film opens, and we have bad CGI, voice-over narration and weird aliens, I thought the movie was going to embrace the nuttiness of the premise, but then it tried to blend in half-baked drama, and it just didn't work.
With comic-book movies, you have to fully commit to your world (tone and all). That's why Christopher Nolan went with a realistic approach to his Batman movies. Regardless of anything, they are tonally consistent. The person above me mentioned Guardians of the Galaxy, and that's a good example of staying true to the premise, tone and reality of the world you are existing in. It seems like James Gunn is going all out, which you have to do if you're going to do a kind of movie like that. Batman is of course much easier to adapt than Green Lantern or Guardians of the Galaxy, but that's why understanding the tone is that much more paramount.
