It is just wrong. CGI is cheap, monsters/blood/explosions don't connect to the set, to the lights; editing is awkward, sometimes a kill is full frontal, gory details and all, and sometimes it happens off-screen; story is as thin as you probably guessed, lines vary from utilitarian to abscond, with maybe an attempt at making it into genre sub-culture but unconvincingly so.
And light/camera work is terrible: if you where willing to actually see the faces, the expressions of the people at work in front of you during one of those "let me explain myself" scene, you are out of luck one time out of two - which is, in these times of all-digital processing, kind of a shame.
And it is full of mistakes, big and small, that you'll surely find a list of on this fine website if only people can be coerced to report on them.
I hope the people who worked that film had fun, where getting paid and all that, and legitimately didn't hold too much hopes for the success of that thing.
As much as I enjoyed Attack the Block, Pacific Rim or Independence Day or other stuff that people should obviously watch without taking it the least bit seriously, well, we are seriously into Anaconda III or various alligator/saurian Terribly Bad CGI siblings territory here; this is definitely no Snakes On A Plane.
