Dear gawd this was awful. Simply miserable. Tedious, dull, stupid, boring - about halfway through I looked over at my girlfriend and she looked miserable and I half-expected her to look at me and beg to go and I realized I didn't have a strong argument against getting up and leaving. We stuck it out because we'd paid our money and to see if anything interesting eventually happened.
The script is dull; the plot vapid; the direction listless; the action scenes inert; Ryan Reynolds is not believable; Blake Lively is almost passable in the beginning before the script gives her nothing to do; the villains suck though Parallax thankfully doesn't have daddy issues. Usually even terrible movies have a couple of good bits, but Green Lantern only almost gives hints of a possibly having a fragment of a good idea in a couple of fleeting spots.
I was miserable the whole time I was in the theater and when we got back to my girlfriend's place, I told her father, "Green Lantern blew goats. I will not be buying the DVD or Blu-ray. I will not be downloading it, so you'll be waiting for it to show up on cable before you see it and I implore you to not bother." He'll probably look at it to see if it's as bad as advertised.
Score: 0.5/10 (that's a half-point). Skip it. I'm not kidding. RUN AWAY!!!
I'm not bothering posting the trailer. Bah.
UPDATE (12/18/11): I picked up a used copy of the Extended Cut on Blu-ray to see if the extras explained why this movie was so terrible. About 30-40 minutes into the PIP mode, it seems like they're pretty satisfied with how it turned out. Self-knowledge was in short supply, it appears.