To start off I don't believe people should make decisions based on what somebody else thinks. That is one of the reasons why I do not like reading reviews half the time and why I avoided this site and several others for almost a week before watching this movie. I didn't want to go into it with any preconceptions because of what somebody who may have very different tastes from myself thought. But now that I have seen it I can say what I personally think was done right and what I think could have been done better. I still encourage you to see the movie if you are at all interested, if only to help progress the medium.
What was done right:
The introduction of the Green Lantern Corps, Hal Jordan: The start of this movie was beautiful. Flying through space being told about the corps and how the rings came into existence was an expected start but I was very pleased with how well done it was. When we first get a glimpse of Hal Jordan he's not as much the one we know from the comics but he does quickly get there. It's true he is a bit more witty than usual but the cockiness is still very much intact and his skills as a pilot are demonstrated very well.
Sinestro: I don't know a single Green Lantern fan that will walk out of this movie without saying that Mark Strong IS Sinestro. The man looks the part and plays it well. You know that he is the lantern that all others look up to and aspire to be like. He could have easily stolen the show and made you forget completely about Hal Jordan.
The casting: I for one think that the casting of this movie is great. As I previously said, Mark Strong was a great choice for Sinestro but Michael Clark Duncan as Killowog and Geoffrey Rush as Tomar Re were good choices as well. Giving Kilowog a deep scary voice and Tomar Re a calm soothing one made the characters almost exactly like I had always imagined them sound in my mind.
What was done okay:
The suit: There have been arguments from the day we all found out that the suit was going to be full CGI and whether it should have been an actual suit that he wore instead. My argument now is the same as it has been since we found that out. The suit itself is an alien entity, it's not made of rubber or spandex and would not transition to a movie well that way. Essentially the suit is going to be a problem regardless of how they do it. Either it looks too fake by being CGI or it becomes too campy but being spandex. There really wasn't a way to do it to satisfy everyone. Plus the mask could not have anything attaching it to Hals face and it would have to stay in place and disappear at a moments notice. I just don't see a way any part of that could have been done to satisfy everybody. At least this way we understand that the ring is what makes the suit and it's not something he has to change into.
The constructs: Some of the constructs are really well done while others do fall a little bit flat. I don't want to spoil anything for anybody but I will say that his first real use of constructs aren't as well made as his first but maybe that was the purpose, because he is still learning how to use the ring.
The corps members: I think all of the members looked great and they all looked very alien. The only downfall is we don't get to see much of them. I suppose it is to be expected since it's not a story about the Green Lantern Corps but a story about Hal Jordan. It just would have been nice to see a few more of them in action.
What was done poorly:
The villains: This is one of the biggest complaints that I have seen is that the movie suffers from too many villains, which I agree to. But by the same token I also have seen people unfamiliar with the Green Lantern saying that they don't understand why they chose these villains as there had to be some better ones out there. I think Parallax was the only logical choice for the first villain in what is supposed to be a trilogy, even if he was a weird smoke monster. Hector Hammond however is where most of the problems stem from. By adding him in it just complicated things that much more for what I see as an unnecessary character.
Comic book purists will find more to complain about I'm sure but I understand that not everything in comics translates as well on screen so I won't get into all that. I will just say that some of the changes were necessary while others weren't and leave the rest for you to decide. I don't think it's as bad of a movie as the critics are making it out to be. It's not Dark Knight good but it's also not Hulk bad, it's nestled comfortably in the middle setting up what could potentially be a great sequel. Go see it and judge your opinion for yourself. Oh and be sure to stay after the credits because that's where the movie does one of the things it absolutely had to do and it does it well.