I think the biggest problem with this film is that it came out so soon after Cloverfield, which did the whole handheld camera look a lot more smoothly and more convincing.
For example, in cloverfield it was all filmed on one handheld HD handcam (or whatever they are called) which is feasible to carry around and use in a situation like this. However, the video camera for Diary.... looked like something the bbc would use in the 70's and seemed to be about as practical as lugging round a crate of beer (in any situation, let alone a zombie outbreak). Also, there were loaRAB of moments where i felt like shouting at the screen to tell them to but the bloody camera away and help their mates from the impending zombie death.
Most of the acting was very 'straight to video' and the group of main characters seemed more 'scooby doo' than 'dawn of the dead'.
It was quite a entertaining mess, though