I liked Green Lantern
quite a lot.
If it had faults - and it did - they were mainly in being way too faithful to the source material. Hal Jordan is not a character with a real strong arc in the comics, though Geoff Johns (and others before him) have tried valiantly to retrofit some kind of character exploration and growth on to him.
They probably invested way too much time and the wrong kind of effort in the "Green Lantern Corps" and Oa. Much as I love the Corps, if you're not prepared to make an all-out Avatar
level technical effort you sure shouldn't be trying to portray humanoids like the Guardians via CGI; the way that Sinestro was done would have been preferable. Too much of the off-world stuff came off as day-glo green crowd scenes with no particular emotional punch.
Hopefully the Justice League
movie, rumored to feature the Jordan incarnation of GL, will rehabilitate the property's movie potential some; the Earth-based Lanterns other than Hal are nonentities and also-rans.