Although this should go without saying, the following is JUST MY OPINION, even though I am not going to start every sentence with IMO.
The superhero movie that Green Lantern most resembles is the original X-Men. It got lots of things right, felt a bit rushed, and left me a tad unsatisfied because I know there is a much richer world waiting to be tapped. This is miles above the Fantastic Four movies, but sadly not near as good as it should have been. Still, 20% on RT is ridiculous. This should be in the 50-60% range at least. If Spider-Man 3 has a fresh rating, I don't see how this doesn't. Thats a movie to be embarrassed by, this is just one that didn't reach its potential. I am not going to say it is as good as TDK. Its not. But it is closer to The Dark Knight than it is to something like Jonah Hex. To avoid the cliche I won't say GL2 could go all "Wrath of Khan," but it could definitely pull an X2.
I really loved the first hour of the film. In fact, about 45 minutes in I was really puzzled at all the negative reviews. Admittedly, the second half was not as strong, but I still believe critics unnecessarily pounced. There were a lot of things that did work in the movie. The costume worked (mask aside). In fact I will go as far as to say the CG costume was the right decision. It looked way better than spandex or leather would have, especially when you add the aliens to the mix. The mask was a bit wonky, but it was much less of a problem in the movie than in still photos. The visuals were for the most part pretty amazing, Ryan Reynolds was surprisingly good. There were good character moments in the first half. Sinestro was great, although not given enough time and the stinger scene was awesome.
My main complaints are that the score was one of the worst of any recent superhero movie. Completely forgettable. The GL Corps were criminally underutilized. Where they were used they were great. A GL2 focusing on them using the set up and designs in this movie would be amazing. The villains were a major letdown. Granted, this storm cloud was better than the one in FF2, but a cloud is still just a cloud. Hammond was also mostly pointless. And the editing of scenes in the second half broke down a bit.
If you are a fan, see the movie and have fun with it. Don't listen to the negative hype. Just enjoy the fact that we finally got a Green Lantern movie that gets a lot right.