Well, as always, I noticed the small stuff. He was in uniform with a beard. Multiple clicks when he ran out of ammo. Didn't see metal shutters on the kid's room windows. And HOW the heck didn't he see that as a trap?
But all and all, I enjoyed the movie except for the ending. I have to agree that it feels like multiple endings were filmed, and it was audience tested for a generic finish.
To make the post appropriately gun related, I think he stuck with the M4 because he was a soldier. Personally, I would have carried SAW. Barring that, a 12 guage. And flares. And for the love of all that's holy, grenades. Lots of grenades. Vampire zombies do tend to dislike grenades. There are NG armories on Lexington Ave, Park Ave and Fifth. NYPD ESU has tons of military grade toys, including APC's. Sigh. Such a target rich environment, and he's entertaining himself with dolls and DVD's. Blah.