1. Compared to what? Maybe this movie only seemed bad until the mediocre X-Men Origins: Wolverine, the disappointing Terminator Salvation, or the out-and-out painful Transformers: Revenge of the Fallen. Maybe people couldn't bare the thought of a fourth horseman of the 80's-action-franchise blockbuster apocalypse. Maybe they all thought: "Well, I didn't want to gouge my eyes out with Oedopus' mother's brooches, so I guess that must have been a decent film."
2. Pick your critics carefully. GIJ:TRoC is currently at 86% on Rotten Tomatoes, but there've only been 7 reviews tabulated. Maybe the pre-screeners were hand-picked to show the film in a favorable early light.
3. The most brilliant marketing plan ever. Maybe we were supposed to hear about how awful this film was, generate a huge amount of negative buzz so that the slightest bit of positive buzz closer to release would seem gut-wrenchingly newsworthy.
Of course, I suppose there's the outside possibility that it merely started out problem-ridden but was greatly improved in post-production. Or that the early negative buzz was undeserved. We'll find out this weekend, I imagine.