So, for the film itself; I'll start by saying that, given Johns' extended stay on the comics title recently, and the way so many readers describe it as "definitive", there's quite a lot of this that goes against his take on the character (Timm and co, of course, having grown up with other comics, though there are things like a mention of the "Honour Lanterns", which is very recent). The Guardians of the Universe all have names, for example, and distinct personalities (one of my favourite parts was Ganthet and co. sadly looking on as a rain of rings descends on Oa after Sinestro disables the battery, which kills every Lantern currently out in space, engaged in combat, etc.). The GL rings and Sinestro's eventual Qwardian ring are fueled by green/yellow "elements" (big glowing crystals) that power the batteries.
This was brought up by the Mighty God King in his review, but this is clearly a product made by people who've known the GL property for a long time, and anticipate an audience that has at least some familiarity; there's basically no attempt to make Hal's discovery of the ring and his journey to Oa some huge eye-opening revelation. He takes these things pretty much in stride so that the chafing-at-the-bit production staff can get to the cool parts. I'm fine with that, though I imagine a film version for a much wider audience will spend more time in the earlier parts.
Overall, though, I really enjoyed it; it's probably the best of the current series of DCU animated films. Not as scattershot in focus and incomplete for fans of the book as New Frontier, lacking the adaptation quibbles I had with Wonder Woman. Like the previous movies, it's got a great voice cast (I often wonder if it's really worth it to shell out for reasonably well-known actors like Tricia Helfer and Michael Madsen to voice side-characters; whatever else might be said of Marvel's DVD casts, they don't suffer for using dedicated voice actors). Meloni delivers a very good Hal (much better than David Boreanaz's). Victor Garber, as you might expect, pretty much steals the movie as Sinestro (actually, he'd be a good live action Sinestro). I was a bit uncertain about Madsen as Kilowog at first, but he grew on me.
Also, while comics fans are just waiting for Sinestro to betray Hal, they managed to genuinely surprise me with the secondary villain.
Narratively, my one real complaint is the very unclear climax; after Sinestro drains the battery, Hal starts hammering away at the 'green element' trying to get it to do something. Then it starts glowing green again and basically turns him into Ion, though none of this is explained.