Green Lantern was just mediocre like so many big budget blockbusters.
The script was weak which in all fairness is hard to do for an origins movie.. about the only good ones that come to mind immediately are Iron Man and Batman Begins (and maybe Spiderman).
The much touted digitally rendered costume was just bad and i ask myself why at all? What's wrong with a good fabric one (that you may spice up with some digital effects)? It looked like cheap rubber and the mask (when he had it on) was just laughable.
The villain was really uninteresting.. maybe in the comics (i didn't read GL though know about major things) Parallax is a more interesting villain but in the movie it felt not as threatening.
We get a short battle sequence where Lanterns fight Parallax and are beaten but it lacks perspective. How strong are Lanterns? It is said there are limited basically only by their willpower but it would have been good to use some of the time of the movie to actually build a up a reputation for the Lanterns, let us see why and how they are the most powerful organization in the universe before they get mauled.
Then we have the standard romantic subplot, the usual array of supporting characters etc.. nothing in this movie was truly original, inspiring or even interesting.
So it became mediocre.. apart from some good SFX scenes and some funny one liners the whole movie fell flat and short of expectations and that's bad for a movie that should have been the kickstarter for the DC movie universe (in the same vein that Iron Man was the kickstarter for the Marvel universe only that Iron Man succeeded).