PROS:
Yay, Green Lantern, and the story of Green Lantern and some Green Lantern stuff on film! I'll list pros later.
CONS:
It didn't contain everything in one film that many fans figured a Green Lantern franchise needed, which apparently gave license to a bunch of black and white thinking, mostly hyperbolic fanboys to pretend to be critics by just being critical, and randomly applying words like "weak" to things that were imperfect, but not actually bad (Much like X3 and SPIDER-MAN 3, actually).
Here are the movie's real cons: The score was only average, there were a few editing/transition issues, the story could have used more background and setup between Hal and his father and there could have been a tad more character work with Hal and the other Jordans, certain training/combat sequences could have been extended for more impact, and Hector Hammond/Parallax probably weren't the best choices of villain to carry the first film. Everything else was pretty solid, including the character portrayals, the acting, the effects, and the action. Its an imperfect film to be sure, but still a very solid one with lots to like about it.
Fanboys just seem to like to be ridiculous about hating this movie. There are legitimate beefs to have with this movie, but half the "cons" listed here and elsewhere are just absurd, half the complaints aren't remotely accurate and some of them don't make any sense.