As a fan of Romero's past zombie films, I was pretty let down by "Diary of the Dead". It has three pretty significant flaws in my humble opinion.
1. No shock: Nothing new or even effective here in terms of scares, effects, or mood. In fact, it falls far short of ever conveying a sense of menace or threat to the cast. Once you've seen one take of fake intestines falling out of a corpse, really, you've seen them all, and we've seen all this before.
2. Fails to get the "first person" perspective: Like them or not, films like "Blair Witch Project" and "Cloverfield" succeeded, in my opinion, in creating a look that made you believe they were being filmed hand-held... that you were getting a live, "as it happened" view. "Diary of the Dead" drops the ball on this in many places. Dialogue is stilted and unnatural. The performances fail to convey the sense that they are living the events, and are pretty flat in general. The shots look too staged and lack any sort of visceral feel. While the narration indicates that the footage has been edited and had music added, the result is a poorly constructed mash of two styles. The narration, music, and edited shots ruin the first person feel, while the attempts at first person come off stiff and unnatural. All in all, really ineffective.
3. Just plain dull: First person pictures have to engender some suspension of disbelief to overcome the obvious thought "why don't you drop the @#$%!! camera and run?" The set up feels really contrived in this film... more so than others. The attempts at social commentary... e.g. government is trying to cover up what's really going on, hunters shooting zombies for fun... are ham-fisted, blunt, and feel tacked-on rather than part of an integrated theme. When the narrator actually asks "Are we really worth saving?" at the end, it was too silly for words. This was the kind of theme Romero put forth excellently in the original "Dawn of the Dead" without having a character blatantly cry out "why do we destroy each other??"
The result is a pretty bland film, with uninteresting performances and boring gore effects. In the special features, Romero makes reference to going back to the original "Night of the Living Dead" with this film. Perhaps he should have done so more literally, 'cause that film still holds up as a masterpiece of terror.