Mind, the movie isn't horrible. It's currently pulling a 24 at Rotten Tomatoes, which seems harsh to me. It's not so much that it's *bad*, as that it relentlessly fails to be *good*.
It's hard to avoid comparing it with X-Men from last week. Green Lantern is, if anything, considerably more true to its source material -- which may be part of the problem. The movie feels consistently like it was written by fanboys. (Which is, admittedly, *very* true to the source material, given that DC is currently run by fanboys.) It isn't willing to wander away from the story a bit in order to be a great movie, and so it isn't.
For example, the main human characters -- Hal, Carol and even Hector Hammond -- are all quite true to their portrayals in the comics. But they come across as consistently flat and two-dimensional here: correct, but not very interesting. The stories are basically right, the motivations are basically right, but it's all paper-thin. (Carol is especially unsatisfying, in a variety of ways that are a little hard to pin down: she's technically following the model of the sometimes-strong character in the comics, but comes across as simply simpering here.) Where the X-Men movie really *added* something, this is just a pastiche of forty years of history. On the technical level they "got it" -- but at a more human level, there's a fundamental lack of depth.
Indeed, even the story is kind of close to right, but manages to be just wrong enough to leech all the interest out. The Big Bad here is Parallax (not a spoiler: this comes in the first five minutes of the film) -- but this Parallax isn't really the soul-possessing demon of the comics, it's just a big monster. (And it is *far* less interesting than the original Parallax, which Geoff Johns and company have conveniently forgotten: a Hal Jordan driven utterly insane through more grief than he could handle.)
If you're a DC fan, you may want to see it anyway: it *is* a whole lot of the four-color comic brought to life semi-faithfully. (Worst I can say there is that they managed to squish the Parallax story into Hal's origin, neatly excising 40 years of in-between continuity.) If not, it's probably not worth bothering: while it's not a complete waste of money, there are better movies out there. So as I said up top: enh...