I guess it wasn't one big thing that disappointed me about Pandorum, but many, many little ones. Only the beginning of which is that this film wants to be Aliens, but for people who don't like to think, follow plotlines, or enjoy good movies. So, SPOILERS AHEAD. (Though, to say that implies there is anything here worth spoiling; there really isn't)
Where do I start? The short version is (get ready) Ben Foster and Dennis Quaid wake as part of the crew on a giant spacecraft which is on it's way to colonize the first Earth-like planet that has been discovered. They emerge from their sleep pods disoriented and lacking most of their memories. They figure out who they are (kind of) and find a control room, but they can't get to the bridge, because the ship has no power. So, Foster has to crawl through a tunnel where he eventually meets a crazy German girl who sounds French and attacks him out of nowhere. She runs off, and Foster finds a hanging dead body, then meets the smaller Boondock Saints brother, who covers himself in grease and is systematically eaten, like something from KFC. Later, Foster is kung-fu attacked by some kind of Pacific Islander space-warrior who doesn't speak English, and meets up with the German French girl again. They are all chased by the creatures from the far superior movie The Descent, who have somehow ended up in space.
MEANWHILE Dennis Quaid finds Cam Gigandet in the same hole that Foster crawled down (!?!?) and pulls him out. Gigandet (who years earlier killed Marissa Cooper, so therefore is not to be trusted) is shaking, nude, and not making sense. Eventually, Quaid calms him some and he begins telling Quaid the story of what happened on the bridge before Foster and Quaid woke. MEANWHILE!! Foster, the German French chick and the Hawaiian Spearfishing Ninja come across a crazy Rastafarian who has made cave paintings all over the walls of his chamber and eventually tells the three that he has lived on the ship for months, eating people and somehow avoiding the monsters. Ooook, still there?
ANYWAY, back in the control room, Gigandet and Quaid are swapping stories and making s'mores. Apparently, one of them killed the bridge crew and one of them may have Pandorum and they may both be the same person, and the first rule of Pandorum is you don't talk about Pandorum.
Oh! I almost forgot! Pandorum, we learn, is some kind of space-madness which turns waking crew members crazy and may be why Gigandet/Quaid had to kill the bridge crew. This info is supposed to give us the impression that the Descent monsters may just be crew members gone mad, but what it really does is add another "plot device" to an already muddled story that certainly didn't need it.
ANYWAY, the Samoan Stick Fighter gets killed somewhere, as does the Rastafarian, and, in the end, Foreign Girl and Foster find Quaid, who is totally batshit with Pandorum and probably killed the bridge crew but is trying to make Foster think HE is crazy, or something. Geez. ANYWAY, turns out the ship was underwater on the destination planet the WHOLE TIME! Whoops! LOLZ!! Everyone laughs and has a tea party, but not really.
In short, I watched this film because I am a fan of Ben Foster, I don't mind Dennis Quaid, and I love sci-fi. Even entertainingly bad sci-fi. Pandorum, however, contains the bad and the sci-fi, without the entertaining.