I enjoyed the GL movie, but I think my biggest issue was that it just tried to introduce too much too quickly, trying to do Hal's origin story with the whole GL Corps, Hammond, and Parallax all at once. I think starting smaller, and then working their way up to bigger things, like the whole GL Corps, and Parallax would have been a better idea.
I agree with this. It was also not very good at establishing Hal as a hero, having him absent for much of the villains' deadly attacks and only showing up late to save one or two people.
But I feel that the biggest problem Green Lantern had was the ill fortune of being released in the same year as Thor, Captain America, and X-Men: First Class. It paled in comparison to those, but I think that if it had come out, say, 5-6 years earlier, compared against the likes of Elektra, Fantastic Four, Superman Returns, and X-Men: The Last Stand, it would've been considered relatively good.